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

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Message-ID: <20170201113536.dig22kidlw6mjoz5@phenom.ffwll.local>
Date:   Wed, 1 Feb 2017 12:35:36 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Peter Senna Tschudin <peter.senna@...labora.co.uk>
Cc:     Archit Taneja <architt@...eaurora.org>,
        ijc+devicetree@...lion.org.uk,
        Fabio Estevam <fabio.estevam@....com>, treding@...dia.com,
        linux-kernel@...r.kernel.org, davem@...emloft.net,
        mark.rutland@....com, ykk@...k-chips.com, martin.donnelly@...com,
        mchehab@....samsung.com, robh+dt@...nel.org, galak@...eaurora.org,
        peter.senna@...il.com, pawel.moll@....com, javier@...hile0.org,
        gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
        shawnguo@...nel.org,
        Peter Senna Tschudin <peter.senna@...labora.com>,
        tiwai@...e.com, linux@...ck-us.net, martyn.welch@...labora.co.uk,
        rmk+kernel@...linux.org.uk, kernel@...gutronix.de,
        daniel.vetter@...ll.ch, jslaby@...e.cz, devicetree@...r.kernel.org,
        linux@...linux.org.uk, linux-arm-kernel@...ts.infradead.org,
        enric.balletbo@...labora.com, dri-devel@...ts.freedesktop.org,
        Jani Nikula <jani.nikula@...ux.intel.com>
Subject: Re: [PATCH V7 3/4] drm/bridge: Add driver for GE B850v3 LVDS/DP++
 Bridge

On Wed, Feb 01, 2017 at 10:58:43AM +0000, Peter Senna Tschudin wrote:
> Hi Archit,
> 
> On 01 February, 2017 10:44 CET, Archit Taneja <architt@...eaurora.org> wrote:
> 
> >
> >
> > 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.
> 
> My concern is that for almost one year now, every time I fix something
> one or two new requests are made. I'm happy to fix the driver, but I
> want a list of the changes that are required to get it upstream, before
> I make more changes. Can we agree on exactly what is preventing this
> driver to get upstream? Then I'll fix it.

I think addressing this edid reading question post-merge is perfectly
fine. Aside, want to keep maintaing your stuff as part of the drm-misc
group, with the drivers-in-misc experiment?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch