Re: [PATCH v2 1/3] drm: Add support for panic message output

["Koenig, Christian" <Christian.Koenig@amd.com>]

Am 13.03.19 um 18:33 schrieb Michel Dänzer:
> [SNIP]
>>>>>>> Copy how? Using a GPU engine?
>>>>>> CPU maybe? Though I suppose that won't work if the buffer isn't CPU
>>>>>> accesible :/
>>>>> Well we do have a debug path for accessing invisible memory with the
>>>>> CPU.
>>>>>
>>>>> E.g. three registers: DATA and auto increment OFFSET_LO/HI. So you can
>>>>> just read/write DATA over and over again if you want to access some
>>>>> memory.
>>>> Right. I assume that'll be very slow, but I guess it could do when the
>>>> memory isn't directly CPU accessible.
>>> Just made a quick test and reading 33423360 bytes (4096x2040x4) using
>>> that interfaces takes about 13 seconds.
>>>
>>> IIRC we don't use the auto increment optimization yet, so that can
>>> probably be improved by a factor of 3 or more.
> I'd assume only writes are needed, no reads.

I've played around with that for a moment and with a bit of optimization 
I can actually get about 20 MB/s write performance out of the debugging 
interface.

This way overwriting a 4K framebuffer would take less than 2 seconds 
using this.

It's not ideal, but I think for a panic screen perfectly reasonable.

Christian.
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