linux-kernel - Re: [PATCH v4 02/11] drm/bridge: it6505: fix aux operation for edid read

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Message-ID: <16833ec2-d6bf-4d7f-9385-1430277fc047@collabora.com>
Date: Thu, 26 Sep 2024 10:10:49 +0200
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
To: Hermes Wu <Hermes.Wu@....com.tw>
Cc: Kenneth Hung <Kenneth.hung@....com.tw>,
 Andrzej Hajda <andrzej.hajda@...el.com>,
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>,
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Allen Chen <allen.chen@....com.tw>,
 "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 02/11] drm/bridge: it6505: fix aux operation for edid
 read

Il 26/09/24 09:47, Hermes Wu ha scritto:
> From: Hermes Wu <Hermes.wu@....com.tw>
> 
> The EDID read operation can reach the maximum size of the AUX FIFO buffer.
> 
> Fixes: b5c84a9edcd4 ("drm/bridge: add it6505 driver")
> Signed-off-by: Hermes Wu <Hermes.wu@....com.tw>
> ---
>   drivers/gpu/drm/bridge/ite-it6505.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/ite-it6505.c b/drivers/gpu/drm/bridge/ite-it6505.c
> index 28a8043229d3..b451d3c2ac1d 100644
> --- a/drivers/gpu/drm/bridge/ite-it6505.c
> +++ b/drivers/gpu/drm/bridge/ite-it6505.c
> @@ -1078,8 +1078,11 @@ static ssize_t it6505_aux_do_transfer(struct it6505 *it6505,
>   	int i, ret_size, ret = 0, request_size;

int fifo_max_sz = (cmd == CMD_AUX_I2C_EDID_READ) ?
		  AUX_FIFO_MAX_SIZE : 4;

which later becomes

int fifo_max_sz = (cmd == CMD_AUX_I2C_EDID_READ || cmd == CMD_AUX_GET_KSV_LIST) ?
		  AUX_FIFO_MAX_SIZE : 4;

otherwise you can do it "the long way"...

if (cmd == .....)
	fifo_max_sz = AUX_FIFO_MAX_SIZE;
else
	fifo_max_sz = 4;

The point is, cmd won't change during iterations, so it's useless to put that if
branch inside of that loop below.

>   
>   	mutex_lock(&it6505->aux_lock);
> -	for (i = 0; i < size; i += 4) {
> -		request_size = min((int)size - i, 4);

		request_size = min_t(int, (int)size - i, fifo_max_sz);

P.S.: Also, I'd consider changing this to a do-while instead...

Cheers,
Angelo

> +	for (i = 0; i < size; ) {
> +		if (cmd == CMD_AUX_I2C_EDID_READ)
> +			request_size = min_t(int, (int)size - i, AUX_FIFO_MAX_SIZE);
> +		else
> +			request_size = min_t(int, (int)size - i, 4);
>   		ret_size = it6505_aux_operation(it6505, cmd, address + i,
>   						buffer + i, request_size,
>   						reply);
> @@ -1088,6 +1091,7 @@ static ssize_t it6505_aux_do_transfer(struct it6505 *it6505,
>   			goto aux_op_err;
>   		}
>   
> +		i += request_size;
>   		ret += ret_size;
>   	}
>