linux-kernel - Re: [PATCH V7 3/4] drm/bridge: Add driver for GE B850v3 LVDS/DP++ Bridge

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Message-ID: <b84ac3b8-3626-3eb2-7293-0affb58b7752@codeaurora.org>
Date:   Wed, 1 Feb 2017 15:14:42 +0530
From:   Archit Taneja <architt@...eaurora.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Peter Senna Tschudin <peter.senna@...labora.com>
Cc:     mark.rutland@....com, daniel.vetter@...ll.ch,
        peter.senna@...il.com, dri-devel@...ts.freedesktop.org,
        tiwai@...e.com, ykk@...k-chips.com, jslaby@...e.cz,
        martyn.welch@...labora.co.uk, mchehab@....samsung.com,
        linux@...linux.org.uk, javier@...hile0.org, treding@...dia.com,
        linux@...ck-us.net, martin.donnelly@...com,
        devicetree@...r.kernel.org, pawel.moll@....com,
        ijc+devicetree@...lion.org.uk,
        Fabio Estevam <fabio.estevam@....com>,
        rmk+kernel@...linux.org.uk, robh+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        galak@...eaurora.org, enric.balletbo@...labora.com,
        akpm@...ux-foundation.org, shawnguo@...nel.org, davem@...emloft.net
Subject: Re: [PATCH V7 3/4] drm/bridge: Add driver for GE B850v3 LVDS/DP++
 Bridge

On 01/30/2017 10:35 PM, Jani Nikula wrote:
> On Sat, 28 Jan 2017, Peter Senna Tschudin <peter.senna@...labora.com> wrote:
>> On Thu, Jan 05, 2017 at 01:18:47PM +0530, Archit Taneja wrote:
>> Hi Archit,
>>
>> Thank you for the comments!
>>
>> [...]
>>>> +	total_size = (block[EDID_EXT_BLOCK_CNT] + 1) * EDID_LENGTH;
>>>> +	if (total_size > EDID_LENGTH) {
>>>> +		kfree(block);
>>>> +		block = kmalloc(total_size, GFP_KERNEL);
>>>> +		if (!block)
>>>> +			return NULL;
>>>> +
>>>> +		/* Yes, read the entire buffer, and do not skip the first
>>>> +		 * EDID_LENGTH bytes.
>>>> +		 */
>>>
>>> Is this the reason why you aren't using drm_do_get_edid()?
>>
>> Yes, for some hw specific reason, it is necessary to read the entire
>> EDID buffer starting from 0, not block by block.
>
> Hrmh, I'm planning on moving the edid override and firmware edid
> mechanisms at the drm_do_get_edid() level to be able to truly and
> transparently use a different edid. Currently, they're only used for
> modes, really, and lead to some info retrieved from overrides, some from
> the real edid. This kind of hacks will bypass the override/firmware edid
> mechanisms then too. :(

It seems like there is a HW issue which prevents them from reading EDID
from an offset. So, I'm not sure if it is a hack or a HW limitation.

One way around this would be to hide the HW requirement in the
get_edid_block func pointer passed to drm_do_get_edid(). This
would, however, result in more i2c reads (equal to # of extension
blocks) than what the patch currently does.

Peter, if you think doing extra EDID reads isn't too costly on your
platform, you could consider using drm_do_get_edid(). If not, I guess
you'll miss out on the additional functionality Jani is going to add
in the future.

Thanks,
Archit

>
> BR,
> Jani.
>
>
>>
>> [...]
>>
>> I fixed all your other suggestions. Thank you!
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@...ts.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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