spi/zynqmp: remove entry that causes a cs glitch

In the public interface for chipselect, there is always an entry commented as "Dummy generic FIFO entry" pushed down to the fifo right after the activate/deactivate command. The dummy entry is 0x0, irregardless if the intention was to activate or deactive the cs. This causes the cs line to glitch rather than beeing activated in the case when there was an activate command.

This has been observed on oscilloscope, and have caused problems for at least one specific flash device type connected to the qspi port. After the change the glitch is gone and cs goes active when intended.

The reason why this worked before (except for the glitch) was because when sending the actual data, the CS bits are once again set. Since most flashes uses mode 0, there is always a half clk period anyway for cs to clk active setup time. If someone would rely on timing from a chip_select call to a transfer_one, it would fail though.

It is unknown why the dummy entry was there in the first place, git log seems to be of no help in this case. The reference manual gives no indication of the necessity of this. In fact the lower 8 bits are a setup (or hold in case of deactivate) time expressed in cycles. So this should not be needed to fulfill any setup/hold timings.

---
 drivers/spi/spi-zynqmp-gqspi.c | 3 ---
 1 file changed, 3 deletions(-)

index 60c4de4e4485..7412a3042a8d 100644
@@ -401,9 +401,6 @@ static void zynqmp_qspi_chipselect(struct spi_device *qspi, bool is_high)
 
 	zynqmp_gqspi_write(xqspi, GQSPI_GEN_FIFO_OFST, genfifoentry);
 
-	/* Dummy generic FIFO entry */
-	zynqmp_gqspi_write(xqspi, GQSPI_GEN_FIFO_OFST, 0x0);
-
 	/* Manually start the generic FIFO command */
 	zynqmp_gqspi_write(xqspi, GQSPI_CONFIG_OFST,
 			zynqmp_gqspi_read(xqspi, GQSPI_CONFIG_OFST) |

2.17.1

Hi Thommy,

The patch

spi/zynqmp: remove entry that causes a cs glitch

has been applied to the spi tree at

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git

All being well this means that it will be integrated into the linux-next tree (usually sometime in the next 24 hours) and sent to Linus during the next merge window (or sooner if it is a bug fix), however if problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing and review of the tree, please engage with people reporting problems and send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they should be sent as incremental updates against current git, existing patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying to this mail.

Thanks,
Mark

From: Thommy Jakobsson <[email protected]>
Date: Mon, 24 Feb 2020 17:26:43 +0100
Subject: [PATCH] spi/zynqmp: remove entry that causes a cs glitch

In the public interface for chipselect, there is always an entry commented as "Dummy generic FIFO entry" pushed down to the fifo right after the activate/deactivate command. The dummy entry is 0x0, irregardless if the intention was to activate or deactive the cs. This causes the cs line to glitch rather than beeing activated in the case when there was an activate command.

This has been observed on oscilloscope, and have caused problems for at least one specific flash device type connected to the qspi port. After the change the glitch is gone and cs goes active when intended.

The reason why this worked before (except for the glitch) was because when sending the actual data, the CS bits are once again set. Since most flashes uses mode 0, there is always a half clk period anyway for cs to clk active setup time. If someone would rely on timing from a chip_select call to a transfer_one, it would fail though.

It is unknown why the dummy entry was there in the first place, git log seems to be of no help in this case. The reference manual gives no indication of the necessity of this. In fact the lower 8 bits are a setup (or hold in case of deactivate) time expressed in cycles. So this should not be needed to fulfill any setup/hold timings.

---
 drivers/spi/spi-zynqmp-gqspi.c | 3 ---
 1 file changed, 3 deletions(-)

index 60c4de4e4485..7412a3042a8d 100644
@@ -401,9 +401,6 @@ static void zynqmp_qspi_chipselect(struct spi_device *qspi, bool is_high)
 
 	zynqmp_gqspi_write(xqspi, GQSPI_GEN_FIFO_OFST, genfifoentry);
 
-	/* Dummy generic FIFO entry */
-	zynqmp_gqspi_write(xqspi, GQSPI_GEN_FIFO_OFST, 0x0);
-
 	/* Manually start the generic FIFO command */
 	zynqmp_gqspi_write(xqspi, GQSPI_CONFIG_OFST,
 			zynqmp_gqspi_read(xqspi, GQSPI_CONFIG_OFST) |

2.20.1