Re: [PATCH 0/3] drm: tinydrm driver for adafruit PiTFT 3.5" touchscreen

["Noralf Trønnes" <noralf@tronnes.org>]

Den 26.10.2018 04.30, skrev Eric Anholt:
> Noralf Trønnes <noralf@tronnes.org> writes:
>
>> Den 25.10.2018 18.29, skrev Eric Anholt:
>>> Eric Anholt <eric@anholt.net> writes:
>>>
>>>> I was going to start working on making the vc4 driver work with
>>>> tinydrm panels, but it turned out tinydrm didn't have the panel I had
>>>> previously bought.  So, last night I ported the fbtft staging
>>>> driver over to DRM.
>>>>
>>>> It seems to work (with DT at
>>>> https://github.com/anholt/linux/commits/drm-misc-next-hx8357d) --
>>>> fbdev works great including rotated, and so does modetest.  However,
>>>> when X11 comes up at 16bpp, I get:
>>>>
>>>> https://photos.app.goo.gl/8tuhzPFFoDGamEfk8
>>>>
>>>> If I have tinydrm set a preferred bpp of 24, X looks great.  Noralf,
>>>> any ideas?
>>> Also, with these patches and the format modifier patch I just sent, mesa
>>> with vc4 is now working with this driver on this branch:
>>>
>>> https://gitlab.freedesktop.org/anholt/mesa/commits/kmsro
>> Ah, nice to see this happening!
>> Getting hw rendering was one of the advantages I saw DRM could provide
>> over fbdev on these displays. Little did I know how complicated graphics
>> was outside fbdev, so I was unable to realise this myself.
>>
>> The current solution to get hw rendering is to have a userspace process
>> that continously copies the framebuffer:
>> https://github.com/tasanakorn/rpi-fbcp
>>
>> It's used by some of the small DIY handheld game consoles that run
>> emulators which requires hw rendering.
>>
>>> Now I wonder how we can improve performance of the SPI updates.
>> At what SPI speed are you running? The datasheet for most of these
>> display controllers list the max speed as 10MHz, but almost all of them
>> can go faster. Some are reported going as high as 70-80MHz. That's for
>> the pixel data transfer, not the commands. tinydrm/mipi-dbi.c sends
>> commands at 10MHz and pixels at full speed (mipi_dbi_spi_cmd_max_speed()).
>> Most panels I have run at 32MHz or 48MHz.
> I copied the DT from the adafruit tree, which has it at 32mhz.  System
> performance seems to be limited by the copy and format conversion I
> think -- in particular, I wonder if we shouldn't be doing our dirty
> copies in our own workqueue.  I haven't managed to get any really good
> profiling data yet, though.
>
> glxgears at 128x128 is nice and smooth, and at 480x320 it's 6fps.
> That's not filling 32mhz of SPI.  On the other hand, I would have
> expected the uncached reads for the 4-to-2 swapped conversion to be able
> to go faster than 3.5mb/sec.  If it's the uncached reads, we could at
> least use NEON for the copy to cached, and probably even do the whole
> conversion in NEON with a bit more thought.
>
> Another option: use a vc4 RCL to do RGBA8888 to RGB565, since that will
> be less pressure on the bus.  But then, I suppose I should just figure
> out what's going on that makes X11 at RGBA8888 break, and fix that so we
> don't even have to do that conversion.
>
> I keep hoping there's some way we could feed output from the DISPSLAVE
> HVS register directly to the SPI master -- FIFO32 gets us close (two
> 16-bit pixels packed next to each other, leftmost in the lower 2 bytes),
> but the need for byte swapping (as opposed to R/B swapping) I think
> makes it impossible.

I just did some speed tests on a 320x240 display at 3 different speeds.
I also tried with byteswapping disabled. Only full updates will benefit
from passing the buffer straight through to SPI. This is because partial
updates are copied to a transfer buffer anyways to minimize SPI transfer
time. No need to transfer things that haven't changed and a memory copy
is far cheaper than a SPI transfer.

SPI at 48MHz:

pi@pi2835:~$ od -An -vtu4 --endian=big 
/sys/bus/spi/devices/spi0.0/of_node/spi-max-frequency
    48000000

pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@XR24 -v
setting mode 320x240-0Hz@XR24 on connectors 28, crtc 30
freq: 24.87Hz
freq: 24.79Hz

pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@RG16 -v
setting mode 320x240-0Hz@RG16 on connectors 28, crtc 30
freq: 26.33Hz
freq: 26.30Hz

disable byteswapping:
pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@RG16 -v
setting mode 320x240-0Hz@RG16 on connectors 28, crtc 30
freq: 28.40Hz
freq: 28.43Hz


SPI at 64MHz (seems to work):

pi@pi2835:~$ od -An -vtu4 --endian=big 
/sys/bus/spi/devices/spi0.0/of_node/spi-max-frequency
    64000000

pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@XR24 -v
setting mode 320x240-0Hz@XR24 on connectors 28, crtc 30
freq: 32.74Hz
freq: 32.69Hz

pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@RG16 -v
setting mode 320x240-0Hz@RG16 on connectors 28, crtc 30
freq: 35.44Hz
freq: 35.19Hz

disabled byteswapping:
pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@RG16 -v
setting mode 320x240-0Hz@RG16 on connectors 28, crtc 30
freq: 39.29Hz
freq: 39.11Hz


SPI at 128MHz (not at all as garbled as I expected):

pi@pi2835:~$ od -An -vtu4 --endian=big 
/sys/bus/spi/devices/spi0.0/of_node/spi-max-frequency
   128000000

pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@XR24 -v
setting mode 320x240-0Hz@XR24 on connectors 28, crtc 30
freq: 48.69Hz
freq: 48.40Hz

pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@RG16 -v
setting mode 320x240-0Hz@RG16 on connectors 28, crtc 30
freq: 53.61Hz
freq: 54.45Hz

disabled byteswapping:
pi@pi2835:~$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 
28:320x240@RG16 -v
setting mode 320x240-0Hz@RG16 on connectors 28, crtc 30
freq: 63.16Hz
freq: 64.19Hz


This is how I disabled byteswapping for this test:

diff --git a/drivers/gpu/drm/tinydrm/mipi-dbi.c 
b/drivers/gpu/drm/tinydrm/mipi-dbi.c
index cb3441e51d5f..fa5d81521485 100644
--- a/drivers/gpu/drm/tinydrm/mipi-dbi.c
+++ b/drivers/gpu/drm/tinydrm/mipi-dbi.c
@@ -228,6 +228,8 @@ static int mipi_dbi_fb_dirty(struct drm_framebuffer *fb,
         DRM_DEBUG("Flushing [FB:%d] x1=%u, x2=%u, y1=%u, y2=%u\n", 
fb->base.id,
                   clip.x1, clip.x2, clip.y1, clip.y2);

+       full = true;
+       swap = false;
         if (!mipi->dc || !full || swap ||
             fb->format->format == DRM_FORMAT_XRGB8888) {
                 tr = mipi->tx_buf;


>> Almost all the time is spent in the SPI transfer, so every hz counts. On
>> the Pi there's byte swapping because the DMA capable SPI controller can't
>> do 16-bit (tinydrm_swab16()). If I remember correctly this has negligible
>> impact on performance.
>>
>> The SPI controller/driver on the Pi has some restrictions on the speeds
>> to choose from because the divisor has to be a multiple of two
>> (bcm2835_spi_transfer_one()).
> That's weird.  My specs say CDIV must be a *power* of two, with lower
> values rounded down.  I guess that means we might be running things
> fast, not slow, though (and at 32mhz out of 250, we should be getting
> the same CDIV).

Yes, that's what the datasheet says.
When fbtft was out-of-tree I distributed a custom kernel with fbtft and
Martin Sperl's DMA capable spi-bcm2708. In that version I also allowed
odd cdiv's with no ill effects reported:
https://github.com/notro/spi-bcm2708/wiki#spi-clock-divider
(see the link to the forum post for details)
Maybe the hw just ignores odd cdiv's, I don't know.

Noralf.