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spi: spi-fsl-lpspi: Fix scldiv calculation

Message ID 20240804113611.83613-1-wahrenst@gmx.net (mailing list archive)
State New, archived
Headers show
Series spi: spi-fsl-lpspi: Fix scldiv calculation | expand

Commit Message

Stefan Wahren Aug. 4, 2024, 11:36 a.m. UTC
The effective SPI clock frequency should never exceed speed_hz
otherwise this might result in undefined behavior of the SPI device.

Currently the scldiv calculation could violate this constraint.
For the example parameters perclk_rate = 24 MHz and speed_hz = 7 MHz,
the function fsl_lpspi_set_bitrate will determine perscale = 0 and
scldiv = 1, which is a effective SPI clock of 8 MHz.

So fix this by rounding up the quotient of perclk_rate and speed_hz.
While this never change within the loop, we can pull this out.

Fixes: 5314987de5e5 ("spi: imx: add lpspi bus driver")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
---
 drivers/spi/spi-fsl-lpspi.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--
2.34.1

Comments

Mark Brown Aug. 8, 2024, 8:12 a.m. UTC | #1
On Sun, 04 Aug 2024 13:36:11 +0200, Stefan Wahren wrote:
> The effective SPI clock frequency should never exceed speed_hz
> otherwise this might result in undefined behavior of the SPI device.
> 
> Currently the scldiv calculation could violate this constraint.
> For the example parameters perclk_rate = 24 MHz and speed_hz = 7 MHz,
> the function fsl_lpspi_set_bitrate will determine perscale = 0 and
> scldiv = 1, which is a effective SPI clock of 8 MHz.
> 
> [...]

Applied to

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

Thanks!

[1/1] spi: spi-fsl-lpspi: Fix scldiv calculation
      commit: 730bbfaf7d4890bd99e637db7767dc68cfeb24e7

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
diff mbox series

Patch

diff --git a/drivers/spi/spi-fsl-lpspi.c b/drivers/spi/spi-fsl-lpspi.c
index 32baa14dfd83..be261ac09df8 100644
--- a/drivers/spi/spi-fsl-lpspi.c
+++ b/drivers/spi/spi-fsl-lpspi.c
@@ -296,7 +296,7 @@  static void fsl_lpspi_set_watermark(struct fsl_lpspi_data *fsl_lpspi)
 static int fsl_lpspi_set_bitrate(struct fsl_lpspi_data *fsl_lpspi)
 {
 	struct lpspi_config config = fsl_lpspi->config;
-	unsigned int perclk_rate, scldiv;
+	unsigned int perclk_rate, scldiv, div;
 	u8 prescale;

 	perclk_rate = clk_get_rate(fsl_lpspi->clk_per);
@@ -313,8 +313,10 @@  static int fsl_lpspi_set_bitrate(struct fsl_lpspi_data *fsl_lpspi)
 		return -EINVAL;
 	}

+	div = DIV_ROUND_UP(perclk_rate, config.speed_hz);
+
 	for (prescale = 0; prescale < 8; prescale++) {
-		scldiv = perclk_rate / config.speed_hz / (1 << prescale) - 2;
+		scldiv = div / (1 << prescale) - 2;
 		if (scldiv < 256) {
 			fsl_lpspi->config.prescale = prescale;
 			break;