dmaengine: stm32-dma: avoid 64-bit division in stm32_dma_get_max_width
PATCH 1/1 dmaengine: stm32-dma: avoid 64-bit division in stm32_dma_get_max_width
From: Arnd Bergmann <[email protected]>
Using the % operator on a 64-bit variable is expensive and can cause a link failure:
arm-linux-gnueabi-ld: drivers/dma/stm32-dma.o: in function `stm32_dma_get_max_width':
stm32-dma.c:(.text+0x170): undefined reference to `__aeabi_uldivmod'
arm-linux-gnueabi-ld: drivers/dma/stm32-dma.o: in function `stm32_dma_set_xfer_param':
stm32-dma.c:(.text+0x1cd4): undefined reference to `__aeabi_uldivmod'
As we know that we just want to check the alignment in stm32_dma_get_max_width(), there is no need for a full division, and using a simple mask is a faster replacement.
In stm32_dma_set_xfer_param(), it is possible to pass a non-power-of-two length, so this does not work. I assume this would in fact be a mistake, and the hardware does not work correctly with a burst of e.g. 5 bytes on a five-byte aligned address. Change this to only allow burst transfers if the address is a multiple of the length, and that length is a power-of-two number.
--- drivers/dma/stm32-dma.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
drivers/dma/stm32-dma.c +3 -3
@@ -280,7 +280,7 @@ static enum dma_slave_buswidth stm32_dma_get_max_width(u32 buf_len, max_width > DMA_SLAVE_BUSWIDTH_1_BYTE) max_width = max_width >>1 ; -if (buf_addr % max_width) +if (buf_addr & (max_width -1 )) max_width =DMA_SLAVE_BUSWIDTH_1_BYTE ;return max_width;
@@ -757,7 +757,7 @@ static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan,* Set memory burst size - burst not possible if address is not aligned on * the address boundary equal to the size of the transfer */ -if (buf_addr % buf_len) +if (!is_power_of_2 (buf_len) || (buf_addr & (buf_len -1 ))) src_maxburst =1 ;else src_maxburst =STM32_DMA_MAX_BURST ;
@@ -813,7 +813,7 @@ static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan,* Set memory burst size - burst not possible if address is not aligned on * the address boundary equal to the size of the transfer */ -if (buf_addr % buf_len) +if (!is_power_of_2 (buf_len) || (buf_addr & (buf_len -1 ))) dst_maxburst =1 ;else dst_maxburst =STM32_DMA_MAX_BURST ;
Signature · 1 line
2.29.2
Hi Arnd,
Thanks for your patch.
Quoted 41 lines · Arnd Bergmann
From: Arnd Bergmann <[email protected]>
Using the % operator on a 64-bit variable is expensive and can cause a link failure:
arm-linux-gnueabi-ld:
drivers/dma/stm32-dma.o: in function `stm32_dma_get_max_width':
stm32-dma.c:(.text+0x170): undefined reference to `__aeabi_uldivmod'
arm-linux-gnueabi-ld:drivers/dma/stm32-dma.o: in function `stm32_dma_set_xfer_param':
stm32-dma.c:(.text+0x1cd4): undefined reference to `__aeabi_uldivmod'As we know that we just want to check the alignment in
stm32_dma_get_max_width(), there is no need for a full division, and using a simple mask is a faster replacement.In
stm32_dma_set_xfer_param(), it is possible to pass a non-power-of-two length, so this does not work. I assume this would in fact be a mistake, and the hardware does not work correctly with a burst of e.g. 5 bytes on a five-byte aligned address. Change this to only allow burst transfers if the address is a multiple of the length, and that length is a power-of-two number.Fixes b20fd5fa310c ("dmaengine: stm32-dma: fix stm32_dma_get_max_width")Signed-off-by Arnd Bergmann <[email protected]>--- drivers/dma/stm32-dma.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)drivers/dma/stm32-dma.c +1
-1@@ -280,7 +280,7 @@ static enum dma_slave_buswidth stm32_dma_get_max_width(u32 buf_len, max_width > DMA_SLAVE_BUSWIDTH_1_BYTE) max_width = max_width >>1 ; -if (buf_addr % max_width) +if (buf_addr & (max_width -1 )) max_width =DMA_SLAVE_BUSWIDTH_1_BYTE ;return max_width;
drivers/dma/stm32-dma.c static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan, · line 760
Quoted 5 lines
drivers/dma/stm32-dma.c +1
-1@@ -757,7 +757,7 @@ static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan,* Set memory burst size - burst not possible if address is not aligned on * the address boundary equal to the size of the transfer */ -if (buf_addr % buf_len) +if (!is_power_of_2 (buf_len) || (buf_addr & (buf_len -1 )))
No need to check !is_power_of_2(buf_len) here. Just after computing src_maxburst,
src_best_burst = stm32_dma_get_best_burst(buf_len, src_maxburst, fifoth, src_addr_width);
The configured burst (src_best_best) already take buf_len into account.
So I would remove !is_power_of_2(buf_len) from the if here and fix the missing space:
CHECK: spaces preferred around that '-' (ctx:WxV)
#68: FILE: drivers/dma/stm32-dma.c:760:
+ if (!is_power_of_2(buf_len) || (buf_addr & (buf_len -1)))
^
drivers/dma/stm32-dma.c static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan, · line 816
Quoted 9 lines
drivers/dma/stm32-dma.c +1
-1src_maxburst =1 ; else src_maxburst =STM32_DMA_MAX_BURST ;@@ -813,7 +813,7 @@ static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan,* Set memory burst size - burst not possible if address is not aligned on * the address boundary equal to the size of the transfer */ -if (buf_addr % buf_len) +if (!is_power_of_2 (buf_len) || (buf_addr & (buf_len -1 )))
Ditto.
drivers/dma/stm32-dma.c static int stm32_dma_set_xfer_param(struct stm32_dma_chan *chan, · line 819
Quoted 4 lines
drivers/dma/stm32-dma.c +0
-0dst_maxburst =1 ; else dst_maxburst =STM32_DMA_MAX_BURST ;
With these fixes, you can add my
Regards,
Amelie