Re: [PATCH] spi: pxa2xx: Set controller->max_transfer_size in dma mode

[Noralf Trønnes <noralf@xxxxxxxxxxx>]

Den 16.10.2019 21.57, skrev Daniel Vetter:
> In DMA mode we have a maximum transfer size, past that the driver
> falls back to PIO (see the check at the top of pxa2xx_spi_transfer_one).
> Falling back to PIO for big transfers defeats the point of a dma engine,
> hence set the max transfer size to inform spi clients that they need
> to do something smarter.
> 
> This was uncovered by the drm_mipi_dbi spi panel code, which does
> large spi transfers, but stopped splitting them after:
> 
> commit e143364b4c1774f68e923a5a0bb0fca28ac25888
> Author: Noralf Trønnes <noralf@xxxxxxxxxxx>
> Date:   Fri Jul 19 17:59:10 2019 +0200
> 
>     drm/tinydrm: Remove tinydrm_spi_max_transfer_size()
> 
> After this commit the code relied on the spi core to split transfers
> into max dma-able blocks, which also papered over the PIO fallback issue.
> 
> Fix this by setting the overall max transfer size to the DMA limit,
> but only when the controller runs in DMA mode.
> 
> Fixes: e143364b4c17 ("drm/tinydrm: Remove tinydrm_spi_max_transfer_size()")
> Cc: Sam Ravnborg <sam@xxxxxxxxxxxx>
> Cc: Noralf Trønnes <noralf@xxxxxxxxxxx>
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
> Reported-and-tested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
> Cc: Daniel Mack <daniel@xxxxxxxxxx>
> Cc: Haojian Zhuang <haojian.zhuang@xxxxxxxxx>
> Cc: Robert Jarzmik <robert.jarzmik@xxxxxxx>
> Cc: Mark Brown <broonie@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-spi@xxxxxxxxxxxxxxx
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
>  drivers/spi/spi-pxa2xx.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c
> index bb6a14d1ab0f..f77201915033 100644
> --- a/drivers/spi/spi-pxa2xx.c
> +++ b/drivers/spi/spi-pxa2xx.c
> @@ -1707,6 +1707,7 @@ static int pxa2xx_spi_probe(struct platform_device *pdev)
>  		} else {
>  			controller->can_dma = pxa2xx_spi_can_dma;
>  			controller->max_dma_len = MAX_DMA_LEN;
> +			controller->max_transfer_size = MAX_DMA_LEN;

As Andy mentioned, ->max_transfer_size is a callback:

struct spi_controller {
	/*
	 * on some hardware transfer / message size may be constrained
	 * the limit may depend on device transfer settings
	 */
	size_t (*max_transfer_size)(struct spi_device *spi);

(I looked at its users and they all return a static or probe time value
so not sure why it's a callback.)

Noralf.

>  		}
>  	}
>  
>