linux-kernel - Re: [PATCH v2 2/2] drm/bridge/synopsys: dsi: handle endianness correctly in dw_mipi_dsi

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Date:   Wed, 10 Jan 2018 15:33:41 +0100
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Brian Norris <briannorris@...omium.org>,
        Archit Taneja <architt@...eaurora.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
Cc:     David Airlie <airlied@...ux.ie>,
        Yannick Fertre <yannick.fertre@...com>,
        Philippe Cornu <philippe.cornu@...com>,
        Vincent Abriou <vincent.abriou@...com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Sean Paul <seanpaul@...omium.org>,
        Nickey Yang <nickey.yang@...k-chips.com>, hl@...k-chips.com,
        linux-rockchip@...ts.infradead.org, mka@...omium.org,
        hoegsberg@...il.com, zyw@...k-chips.com, xbl@...k-chips.com
Subject: Re: [PATCH v2 2/2] drm/bridge/synopsys: dsi: handle endianness
 correctly in dw_mipi_dsi_write()

On 09.01.2018 21:32, Brian Norris wrote:
> We're filling the "remainder" word with little-endian data, then writing
> it out to IO registers with endian-correcting writel(). That probably
> won't work on big-endian systems.
>
> Let's mark the "remainder" variable as LE32 (since we fill it with
> memcpy()) and do the swapping explicitly.
>
> Some of this function could be done more easily without memcpy(), but
> the unaligned "remainder" case is a little hard to do without
> potentially overrunning 'tx_buf', so I just applied the same solution in
> all cases (memcpy() + le32_to_cpu()).
>
> Tested only on a little-endian system.
>
> Signed-off-by: Brian Norris <briannorris@...omium.org>
> ---
>  drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> index ed91e32ee43a..90f13df6f106 100644
> --- a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> +++ b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> @@ -360,18 +360,18 @@ static int dw_mipi_dsi_write(struct dw_mipi_dsi *dsi,
>  {
>  	const u8 *tx_buf = packet->payload;
>  	int len = packet->payload_length, pld_data_bytes = sizeof(u32), ret;
> -	u32 remainder;
> +	__le32 word;
>  	u32 val;
>  
>  	while (len) {
>  		if (len < pld_data_bytes) {
> -			remainder = 0;
> -			memcpy(&remainder, tx_buf, len);
> -			dsi_write(dsi, DSI_GEN_PLD_DATA, remainder);
> +			word = 0;
> +			memcpy(&word, tx_buf, len);
> +			dsi_write(dsi, DSI_GEN_PLD_DATA, le32_to_cpu(word));
>  			len = 0;
>  		} else {
> -			memcpy(&remainder, tx_buf, pld_data_bytes);
> -			dsi_write(dsi, DSI_GEN_PLD_DATA, remainder);
> +			memcpy(&word, tx_buf, pld_data_bytes);
> +			dsi_write(dsi, DSI_GEN_PLD_DATA, le32_to_cpu(word));
>  			tx_buf += pld_data_bytes;
>  			len -= pld_data_bytes;
>  		}
> @@ -386,9 +386,9 @@ static int dw_mipi_dsi_write(struct dw_mipi_dsi *dsi,
>  		}
>  	}
>  
> -	remainder = 0;
> -	memcpy(&remainder, packet->header, sizeof(packet->header));
> -	return dw_mipi_dsi_gen_pkt_hdr_write(dsi, remainder);
> +	word = 0;
> +	memcpy(&word, packet->header, sizeof(packet->header));
> +	return dw_mipi_dsi_gen_pkt_hdr_write(dsi, le32_to_cpu(word));

You could create and use appropriate helper, lets say:

u32 le_to_cpup(const u8 *p, int count)
{
    __le32 r = 0;

    memcpy(&r, p, count);
    return le32_to_cpu(r);
}

With or without this change:
Reviewed-by: Andrzej Hajda <a.hajda@...sung.com>

 --
Regards
Andrzej


>  }
>  
>  static ssize_t dw_mipi_dsi_host_transfer(struct mipi_dsi_host *host,