mmc: sdhci: Properly set DMA mask

Review

Quoted 1 line

(quoted diff) +1 -0

+	u64 dma_mask = dma_get_mask(dev);

This is not a driver API. I think what you want is dma_get_required_mask to query the mask. But in that case you still need to always actually set a mask in the driver as well.

Something like this patch:

drivers/mmc/host/sdhci.c +7 -0

index a22e11a65658..36c61778d8f3 100644
@@ -3500,6 +3500,13 @@ static int sdhci_set_dma_mask(struct sdhci_host *host)
 	struct device *dev = mmc_dev(mmc);
 	int ret = -EINVAL;
 
+	/*
+	 * Systems that can't address more than 32-bits do not need to use
+	 * 64-bit addressing mode, even if the device supports it.
+	 */
+	if (dma_get_required_mask(dev) <= DMA_BIT_MASK(32))
+		host->flags &= ~SDHCI_USE_64_BIT_DMA;
+
 	if (host->quirks2 & SDHCI_QUIRK2_BROKEN_64_BIT_DMA)
 		host->flags &= ~SDHCI_USE_64_BIT_DMA;
 

Quoted 5 lines · Christoph Hellwig

Review

Quoted 1 line

(quoted diff) +1 -0

+	u64 dma_mask = dma_get_mask(dev);

This is not a driver API. I think what you want is dma_get_required_mask to query the mask. But in that case you still need to always actually set a mask in the driver as well.

That's slightly different from what I want to do here. The purpose of this part of the patch is that when the SDHCI hardware supports only 32 bits of address space, then we want to prevent 64-bit addressing mode from being used because it isn't useful.

So what we do want to check here is the DMA mask configured by the driver (or the default set by the bus, or wherever it came from). dma_get_required_mask() returns the DMA mask required to address all of system memory. That would perhaps be a useful additional check, but it's orthogonal to what I'm trying to do here.

Is there something else appropriate that I could use to query the DMA mask set for a device?

From: Thierry Reding <[email protected]>

The implementation of sdhci_set_dma_mask() is conflating two things: on one hand it uses the SDHCI_USE_64_BIT_DMA flag to determine whether or not to use the 64-bit addressing capability of the controller and on the other hand it also uses that flag to set a DMA mask for the controller's parent device.

However, a controller supporting 64-bit addressing doesn't mean that it needs to support addressing 64 bits of address range. It's perfectly acceptable to use 64-bit addressing for a 32-bit address range or even smaller, even if that makes little sense, considering the extra overhead of the 64-bit addressing descriptors.

But it is fairly common for hardware to support somewhere between 32 and 64 bits of address range. Tegra124 and Tegra210, for example, support 34 bits and the newer Tegra186 and Tegra194 support 40 bits. The latter can also use an IOMMU for address translation, which has an input address range of 48 bits. This causes problems with the current algorithm in the SDHCI core for choosing the DMA mask. If the DMA mask is set to 64 bits, the DMA memory allocations can (and usually do because the allocator starts from the top) end up beyond the 40 bit boundary addressable by the SDHCI controller, causing IOMMU faults.

For Tegra specifically this problem is currently worked around by setting the SDHCI_QUIRK2_BROKEN_64_BIT_DMA quirk. This causes the DMA mask to always be set to 32 bits and therefore all allocations will fit within the range addressable by the controller.

This commit reworks the code in sdhci_set_dma_mask() to fix the above issue. The rationale behind this change is that the SDHCI controller driver should be the authoritative source of the DMA mask setting. The SDHCI core has no way of knowing what the individual SDHCI controllers are capable of. So instead of overriding the DMA mask depending on whether or not 64-bit addressing mode is used, the DMA mask is only modified if absolutely necessary. On one hand, if the controller can only address 32 bits of memory or less, we disable use of 64-bit addressing mode because it is not needed. On the other hand, we also want to make sure that if we don't support 64-bit addressing mode, such as in the case where the BROKEN_64_BIT_DMA quirk is set, we do restrict the DMA mask to fit the capabilities. The latter is an inconsistency by the driver, so we warn about it to make sure it will be addressed in the driver.

Signed-off-by Thierry Reding <[email protected]>

---
 drivers/mmc/host/sdhci.c | 36 ++++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 14 deletions(-)

drivers/mmc/host/sdhci.c +22 -14

index 7c6c93e85b7e..01f81e96be23 100644
@@ -3499,27 +3499,35 @@ static int sdhci_set_dma_mask(struct sdhci_host *host)
 {
 	struct mmc_host *mmc = host->mmc;
 	struct device *dev = mmc_dev(mmc);
-	int ret = -EINVAL;
+	u64 dma_mask = dma_get_mask(dev);
+	u64 dma32 = DMA_BIT_MASK(32);
+	int ret = 0;
 
 	if (host->quirks2 & SDHCI_QUIRK2_BROKEN_64_BIT_DMA)
 		host->flags &= ~SDHCI_USE_64_BIT_DMA;
 
-	/* Try 64-bit mask if hardware is capable  of it */
-	if (host->flags & SDHCI_USE_64_BIT_DMA) {
-		ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64));
-		if (ret) {
-			pr_warn("%s: Failed to set 64-bit DMA mask.\n",
-				mmc_hostname(mmc));
-			host->flags &= ~SDHCI_USE_64_BIT_DMA;
-		}
+	/*
+	 * Hardware that can't address more than the 32-bit address range does
+	 * not need to use 64-bit addressing mode, even if it supports it.
+	 */
+	if ((host->flags & SDHCI_USE_64_BIT_DMA) && (dma_mask <= dma32)) {
+		pr_debug("%s: controller needs addresses <= 32-bits\n",
+			mmc_hostname(mmc));
+		host->flags &= ~SDHCI_USE_64_BIT_DMA;
 	}
 
-	/* 32-bit mask as default & fallback */
-	if (ret) {
-		ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
+	/*
+	 * If the hardware doesn't support 64-bit addressing, make sure to
+	 * restrict the DMA mask so we don't get buffers allocated beyond the
+	 * 32-bit boundary.
+	 */
+	if (!(host->flags & SDHCI_USE_64_BIT_DMA) && (dma_mask > dma32)) {
+		WARN(1, "64-bit DMA not supported, DMA mask %llx\n", dma_mask);
+
+		ret = dma_set_mask_and_coherent(dev, dma32);
 		if (ret)
-			pr_warn("%s: Failed to set 32-bit DMA mask.\n",
-				mmc_hostname(mmc));
+			pr_warn("%s: failed to set 32-bit DMA mask: %d\n",
+				mmc_hostname(mmc), ret);
 	}
 
 	return ret;

Quoted 29 lines · Thierry Reding

From: Thierry Reding <[email protected]>

The implementation of sdhci_set_dma_mask() is conflating two things: on one hand it uses the SDHCI_USE_64_BIT_DMA flag to determine whether or not to use the 64-bit addressing capability of the controller and on the other hand it also uses that flag to set a DMA mask for the controller's parent device.

However, a controller supporting 64-bit addressing doesn't mean that it needs to support addressing 64 bits of address range. It's perfectly acceptable to use 64-bit addressing for a 32-bit address range or even smaller, even if that makes little sense, considering the extra overhead of the 64-bit addressing descriptors.

But it is fairly common for hardware to support somewhere between 32 and 64 bits of address range. Tegra124 and Tegra210, for example, support 34 bits and the newer Tegra186 and Tegra194 support 40 bits. The latter can also use an IOMMU for address translation, which has an input address range of 48 bits. This causes problems with the current algorithm in the SDHCI core for choosing the DMA mask. If the DMA mask is set to 64 bits, the DMA memory allocations can (and usually do because the allocator starts from the top) end up beyond the 40 bit boundary addressable by the SDHCI controller, causing IOMMU faults.

For Tegra specifically this problem is currently worked around by setting the SDHCI_QUIRK2_BROKEN_64_BIT_DMA quirk. This causes the DMA mask to always be set to 32 bits and therefore all allocations will fit within the range addressable by the controller.

Quoted 14 lines

This commit reworks the code in sdhci_set_dma_mask() to fix the above issue. The rationale behind this change is that the SDHCI controller driver should be the authoritative source of the DMA mask setting. The SDHCI core has no way of knowing what the individual SDHCI controllers are capable of. So instead of overriding the DMA mask depending on whether or not 64-bit addressing mode is used, the DMA mask is only modified if absolutely necessary. On one hand, if the controller can only address 32 bits of memory or less, we disable use of 64-bit addressing mode because it is not needed. On the other hand, we also want to make sure that if we don't support 64-bit addressing mode, such as in the case where the BROKEN_64_BIT_DMA quirk is set, we do restrict the DMA mask to fit the capabilities. The latter is an inconsistency by the driver, so we warn about it to make sure it will be addressed in the driver.

sdhci_set_dma_mask() was added because people did want sdhci to set the DMA mask. Also using 64-bit DMA even with 32-bit systems has the advantage of reducing exposure to problems i.e. the same logic is used with the same SoC irrespective of whether or not it is in 32-bit compatibility mode. So the policy for sdhci is always to use 64-bit DMA if it is supported.

I suggest we add a new sdhci op for ->set_dma_mask() and call that instead of sdhci_set_dma_mask() if it is not NULL.

-----Original Message-----
From: Thierry Reding [mailto:[email protected]]
Sent: Thursday, January 10, 2019 6:02 PM
To: Hunter, Adrian <[email protected]>
Cc: Ulf Hansson <[email protected]>; Jonathan Hunter <[email protected]>; Sowjanya Komatineni <[email protected]>; Krishna Reddy <[email protected]>; linux- [email protected]; [email protected]; linux- [email protected]
Subject: Re: [PATCH 1/2] mmc: sdhci: Properly set DMA mask

Quoted 15 lines · Adrian Hunter

Quoted 14 lines · Thierry Reding

From: Thierry Reding <[email protected]>

The implementation of sdhci_set_dma_mask() is conflating two things:
on one hand it uses the SDHCI_USE_64_BIT_DMA flag to determine whether or not to use the 64-bit addressing capability of the controller and on the other hand it also uses that flag to set a DMA mask for the controller's parent device.

However, a controller supporting 64-bit addressing doesn't mean that it needs to support addressing 64 bits of address range. It's perfectly acceptable to use 64-bit addressing for a 32-bit address range or even smaller, even if that makes little sense, considering the extra overhead of the 64-bit addressing descriptors.

Quoted 10 lines

Quoted 10 lines

But it is fairly common for hardware to support somewhere between 32 and
64 bits of address range. Tegra124 and Tegra210, for example, support 34 bits and the newer Tegra186 and Tegra194 support 40 bits. The latter can also use an IOMMU for address translation, which has an input address range of 48 bits. This causes problems with the current algorithm in the SDHCI core for choosing the DMA mask. If the DMA mask is set to 64 bits, the DMA memory allocations can (and usually do because the allocator starts from the top) end up beyond the 40 bit boundary addressable by the SDHCI controller, causing IOMMU

faults.

Quoted 21 lines

Quoted 6 lines

For Tegra specifically this problem is currently worked around by setting the SDHCI_QUIRK2_BROKEN_64_BIT_DMA quirk. This causes the DMA mask to always be set to 32 bits and therefore all allocations will fit within the range addressable by the controller.

Quoted 14 lines

This commit reworks the code in sdhci_set_dma_mask() to fix the above issue. The rationale behind this change is that the SDHCI controller driver should be the authoritative source of the DMA mask setting. The SDHCI core has no way of knowing what the individual SDHCI controllers are capable of. So instead of overriding the DMA mask depending on whether or not 64-bit addressing mode is used, the DMA mask is only modified if absolutely necessary. On one hand, if the controller can only address 32 bits of memory or less, we disable use of 64-bit addressing mode because it is not needed. On the other hand, we also want to make sure that if we don't support 64-bit addressing mode, such as in the case where the BROKEN_64_BIT_DMA quirk is set, we do restrict the DMA mask to fit the capabilities. The latter is an inconsistency by the driver, so we warn about it to make sure it will be addressed in the driver.

Quoted 5 lines

sdhci_set_dma_mask() was added because people did want sdhci to set the DMA mask. Also using 64-bit DMA even with 32-bit systems has the advantage of reducing exposure to problems i.e. the same logic is used with the same SoC irrespective of whether or not it is in 32-bit compatibility mode. So the policy for sdhci is always to use 64-bit DMA if it

is supported.

Quoted 3 lines

I suggest we add a new sdhci op for ->set_dma_mask() and call that instead of sdhci_set_dma_mask() if it is not NULL.

Some drivers are already doing something similar by overriding the DMA mask again in ->enable_dma(). I had briefly considered doing that for Tegra, but after thinking about it, it just became clear to me that we shouldn't need to override this in every driver. I just don't think it's correct for the MMC core to muck with the DMA mask. Just because the hardware supports the SDHCI 64-bit addressing mode doesn't mean that all
64 bits can be addressed by the hardware. The DMA mask defines what the valid address range is for the device and it's already conventional for drivers to set this early in their ->probe() implementation (or have the bus set it up). It seems wasteful to have to redo that in a custom callback.