linux-kernel - Re: [PATCH v2 4/6] drm/msm/dsi: add a comment to explain pkt_per

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Message-ID: <551382fc-4256-4e2f-974c-9b0f6814a8f8@linaro.org>
Date:   Sun, 3 Dec 2023 00:23:24 +0200
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Jonathan Marek <jonathan@...ek.ca>
Cc:     freedreno@...ts.freedesktop.org, Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Jiasheng Jiang <jiasheng@...as.ac.cn>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/6] drm/msm/dsi: add a comment to explain pkt_per_line
 encoding

On 16/11/2023 20:45, Jonathan Marek wrote:
> On 11/15/23 2:38 AM, Dmitry Baryshkov wrote:
>> On Wed, 15 Nov 2023 at 01:00, Jonathan Marek <jonathan@...ek.ca> wrote:
>>>
>>> Make it clear why the pkt_per_line value is being "divided by 2".
>>>
>>> Signed-off-by: Jonathan Marek <jonathan@...ek.ca>
>>> ---
>>>   drivers/gpu/drm/msm/dsi/dsi_host.c | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c 
>>> b/drivers/gpu/drm/msm/dsi/dsi_host.c
>>> index 66f198e21a7e..842765063b1b 100644
>>> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
>>> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
>>> @@ -877,6 +877,8 @@ static void dsi_update_dsc_timing(struct 
>>> msm_dsi_host *msm_host, bool is_cmd_mod
>>>          /* DSI_VIDEO_COMPRESSION_MODE & DSI_COMMAND_COMPRESSION_MODE
>>>           * registers have similar offsets, so for below common code use
>>>           * DSI_VIDEO_COMPRESSION_MODE_XXXX for setting bits
>>> +        *
>>> +        * pkt_per_line is log2 encoded, >>1 works for supported 
>>> values (1,2,4)
>>>           */
>>>          reg |= 
>>> DSI_VIDEO_COMPRESSION_MODE_CTRL_PKT_PER_LINE(pkt_per_line >> 1);
>>
>> Should we switch to ffs() or fls() instead?
>>
> 
> Just a ffs() on its own can be confusing as well (without the 
> information that only powers of two are possible), I think like this is 
> better.

Sounds fair. Could you please then add `if (pkt_per_line > 4) 
drm_warn("pkt_per_line too big");`

With that in place:

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

> 
>>>          reg |= 
>>> DSI_VIDEO_COMPRESSION_MODE_CTRL_EOL_BYTE_NUM(eol_byte_num);
>>> -- 
>>> 2.26.1
>>>
>>
>>

-- 
With best wishes
Dmitry