linux-kernel - Re: [PATCH v15 1/2] drm/tegra: dc: Support memory bandwidth management

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Message-ID: <49a69c69-44e7-741f-b86b-ef4fe83c76b1@gmail.com>
Date:   Wed, 17 Mar 2021 01:56:38 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Matt Merhar <mattmerhar@...tonmail.com>,
        Peter Geis <pgwipeout@...il.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        linux-tegra@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v15 1/2] drm/tegra: dc: Support memory bandwidth
 management

15.03.2021 21:39, Dmitry Osipenko пишет:
>>> +	/*
>>> +	 * Horizontal downscale needs a lower memory latency, which roughly
>>> +	 * depends on the scaled width.  Trying to tune latency of a memory
>>> +	 * client alone will likely result in a strong negative impact on
>>> +	 * other memory clients, hence we will request a higher bandwidth
>>> +	 * since latency depends on bandwidth.  This allows to prevent memory
>>> +	 * FIFO underflows for a large plane downscales, meanwhile allowing
>>> +	 * display to share bandwidth fairly with other memory clients.
>>> +	 */
>>> +	if (src_w > dst_w)
>>> +		mul = (src_w - dst_w) * bpp / 2048 + 1;
>>> +	else
>>> +		mul = 1;
>> [...]
>>
>> One point is unexplained yet: why is the multiplier proportional to a
>> *difference* between src and dst widths? Also, I would expect max (worst
>> case) is pixclock * read_size when src_w/dst_w >= read_size.
> IIRC, the difference gives a more adequate/practical result than the
> proportion. Although, downstream driver uses proportion. I'll try to
> revisit this for the next version of the patch.

I tried to re-test everything and can't reproduce problems that existed
previously. We didn't have a finished memory drivers back then and I
think that Tegra30 latency tuning support and various Tegra20 changes
fixed those problems. I'll remove this hunk in the next version.