[PATCH] drm/dbi: Fix SPI Type 1 (9-bit) transfer

[Noralf Trønnes]

Den 03.07.2020 16.13, skrev Paul Cercueil:
> The function mipi_dbi_spi1_transfer() will transfer its payload as 9-bit
> data, the 9th (MSB) bit being the data/command bit. In order to do that,
> it unpacks the 8-bit values into 16-bit values, then sets the 9th bit if
> the byte corresponds to data, clears it otherwise. The 7 MSB are
> padding. The array of now 16-bit values is then passed to the SPI core
> for transfer.
> 
> This function was broken since its introduction, as the length of the
> SPI transfer was set to the payload size before its conversion, but the
> payload doubled in size due to the 8-bit -> 16-bit conversion.
> 
> Fixes: 02dd95fe3169 ("drm/tinydrm: Add MIPI DBI support")
> Cc: <stable at vger.kernel.org> # 4.10

The code was moved to drm_mipi_dbi.c in 5.4 so this patch won't apply
before that.

> Signed-off-by: Paul Cercueil <paul at crapouillou.net>
> ---

Thanks for fixing this, clearly I didn't test this. Probably because the
aux spi ip block on the Raspberry Pi that can do 9 bit didn't have a
driver at the time. Did you actually test this or was it spotted reading
the code?

Reviewed-by: Noralf Trønnes <noralf at tronnes.org>

>  drivers/gpu/drm/drm_mipi_dbi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_mipi_dbi.c b/drivers/gpu/drm/drm_mipi_dbi.c
> index bb27c82757f1..bf7888ad9ad4 100644
> --- a/drivers/gpu/drm/drm_mipi_dbi.c
> +++ b/drivers/gpu/drm/drm_mipi_dbi.c
> @@ -923,7 +923,7 @@ static int mipi_dbi_spi1_transfer(struct mipi_dbi *dbi, int dc,
>  			}
>  		}
>  
> -		tr.len = chunk;
> +		tr.len = chunk * 2;
>  		len -= chunk;
>  
>  		ret = spi_sync(spi, &m);
>