linux-kernel - Re: [PATCH v2 0/4] drm/tiny: Add driver for Solomon SSD1307 OLED displays

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Date:   Tue, 8 Feb 2022 16:32:10 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Noralf Trønnes <noralf@...nnes.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maxime Ripard <maxime@...no.tech>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Lee Jones <lee.jones@...aro.org>
Subject: Re: [PATCH v2 0/4] drm/tiny: Add driver for Solomon SSD1307 OLED
 displays

Hello Mark,

On 2/8/22 16:18, Mark Brown wrote:
> On Tue, Feb 08, 2022 at 04:10:49PM +0100, Javier Martinez Canillas wrote:
>> On 2/8/22 15:19, Geert Uytterhoeven wrote:
> 
>>>   - "time ls" on the serial console (no files in the current directory,
>>>     so nothing to print) increases from 0.86s to 1.92s, so the system is
>>>     more loaded.  As ssd1307fb relied on deferred I/O too, the slowdown
>>>     might be (partly) due to redrawing of the visual artefacts
>>>     mentioned above.
> 
>> I was trying to first have the driver and then figure out how to optimize
>> it. For v3 I'm using regmap to access instead of the I2C layer directly.
> 
>> I noticed that this is even slower but it makes the driver more clean and
>> allows to support both I2C and SPI (untested but will include it as a WIP).
> 
> I wouldn't have expected regmap to add huge overhead relative to I2C,
> partly predicated on I2C being rather slow itself.  There will be some
> overhead for concurrency protection and data marshalling but for I2C
> clocked at normal speeds it's surprising.

Thanks for chiming in. That's good to know, I'll investigate more then.

Probably I was wrongly blaming regmap while it was another change that
is causing the display to be refreshed at a slower rate than before.

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat