Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <a1136c32-0c29-3803-da15-59b40fcf333e@gmail.com>
Date:   Wed, 12 Apr 2023 12:39:46 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.6.1
Subject: Re: [PATCH v3 1/9] drm/mediatek: dp: Cache EDID for eDP panel
Content-Language: en-US
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>, chunkuang.hu@kernel.org
Cc:     p.zabel@pengutronix.de, airlied@gmail.com, daniel@ffwll.ch,
        dri-devel@lists.freedesktop.org,
        linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kernel@collabora.com,
        wenst@chromium.org
References: <20230404104800.301150-1-angelogioacchino.delregno@collabora.com>
 <20230404104800.301150-2-angelogioacchino.delregno@collabora.com>
 <09c61b94-1ed1-eb72-9682-1f1f203f6f63@gmail.com>
 <783c03af-fc88-96c8-c6fc-6f02051dc6b1@collabora.com>
From:   Matthias Brugger <matthias.bgg@gmail.com>
In-Reply-To: <783c03af-fc88-96c8-c6fc-6f02051dc6b1@collabora.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk


On 12/04/2023 10:06, AngeloGioacchino Del Regno wrote:
> Il 12/04/23 09:08, Matthias Brugger ha scritto:
>>
>>
>> On 04/04/2023 12:47, AngeloGioacchino Del Regno wrote:
>>> Since eDP panels are not removable it is safe to cache the EDID:
>>> this will avoid a relatively long read transaction at every PM
>>> resume that is unnecessary only in the "special" case of eDP,
>>> hence speeding it up a little, as from now on, as resume operation,
>>> we will perform only link training.
>>>
>>> Signed-off-by: AngeloGioacchino Del Regno 
>>> <angelogioacchino.delregno@collabora.com>
>>> ---
>>>   drivers/gpu/drm/mediatek/mtk_dp.c | 11 ++++++++++-
>>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/mediatek/mtk_dp.c 
>>> b/drivers/gpu/drm/mediatek/mtk_dp.c
>>> index 1f94fcc144d3..84f82cc68672 100644
>>> --- a/drivers/gpu/drm/mediatek/mtk_dp.c
>>> +++ b/drivers/gpu/drm/mediatek/mtk_dp.c
>>> @@ -118,6 +118,7 @@ struct mtk_dp {
>>>       const struct mtk_dp_data *data;
>>>       struct mtk_dp_info info;
>>>       struct mtk_dp_train_info train_info;
>>> +    struct edid *edid;
>>>       struct platform_device *phy_dev;
>>>       struct phy *phy;
>>> @@ -1993,7 +1994,11 @@ static struct edid *mtk_dp_get_edid(struct drm_bridge 
>>> *bridge,
>>>           usleep_range(2000, 5000);
>>>       }
>>> -    new_edid = drm_get_edid(connector, &mtk_dp->aux.ddc);
>>> +    /* eDP panels aren't removable, so we can return a cached EDID. */
>>> +    if (mtk_dp->edid && mtk_dp->bridge.type == DRM_MODE_CONNECTOR_eDP)

Maybe better like this:
if (mtk_dp->bridge.type == DRM_MODE_CONNECTOR_eDP && mtk_dp->edid)

To in sync with the if statement below. Anyway we are only concerned if it's an 
eDP so check that first (and hope the compiler will do so as well ;)

With that:
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>

>>> +        new_edid = drm_edid_duplicate(mtk_dp->edid);
>>> +    else
>>> +        new_edid = drm_get_edid(connector, &mtk_dp->aux.ddc);
>>
>> Maybe it would make sense to add a macro for the check of mtk_dp->bridge.type 
>> == DRM_MODE_CONNECTOR_eDP
>> it would make the code more readable.
>>
> 
> I had the same idea... but then avoided that because in most (if not all?) of the
> DRM drivers (at least, the one I've read) this check is always open coded, so I
> wrote it like that for consistency and nothing else.
> 
> I have no strong opinions on that though!
> 

I think the only reasonable solution would be a macro like:
DRM_CONNECTOR_MODE_IS(mtk_dp->bridge.type, eDP) which in the end is longer then 
open-code it, so probably just leave it as it is.

>>>       /*
>>>        * Parse capability here to let atomic_get_input_bus_fmts and
>>> @@ -2022,6 +2027,10 @@ static struct edid *mtk_dp_get_edid(struct drm_bridge 
>>> *bridge,
>>>           drm_atomic_bridge_chain_post_disable(bridge, connector->state->state);
>>>       }
>>> +    /* If this is an eDP panel and the read EDID is good, cache it for later */
>>> +    if (mtk_dp->bridge.type == DRM_MODE_CONNECTOR_eDP && !mtk_dp->edid && 
>>> new_edid)
>>> +        mtk_dp->edid = drm_edid_duplicate(new_edid);
>>> +
>>
>> How about putting this in an else if branch of mtk_dp_parse_capabilities. At 
>> least we could get rid of the check regarding if new_edid != NULL.
>>
>> I was thinking on how to put both if statements in one block, but I think the 
>> problem is, that we would leak memory if the capability parsing failes due to 
>> the call to drm_edid_duplicate(). Correct?
>>
> 
> Correct. The only other "good" place would be in the `if (new_edid)` conditional,
> but that wouldn't be as readable as it is right now...
> 
> Cheers,
> Angelo
>