vga_pw sysfs file
joel at jms.id.au
Wed Nov 17 11:57:11 AEDT 2021
On Tue, 16 Nov 2021 at 14:40, Oskar Senft <osk at google.com> wrote:
> Hi everyone
> I'm currently upgrading from Linux kernel 4.10 to to 5.15 and found
> that uart_render_controller  can no longer detect whether the host
> is driving the Aspeed's VGA output.
> I tracked it down to a difference on how the VGA scratch register used
> to be read by aspeed-bmc-misc.c vs. how it's now read by the
> aspeed_gfx_drv.c driver:
> - In aspeed-bmc-misc.c we ended up with the full contents of SCU50 in sysfs.
> - With aspeed_gfx_drv.c we now only get bit 0 . Unfortunately, at
> least in my BIOS, the VGA scratch register never has bit 0 set.
> In uart_render_controller, however, we're checking whether the bottom
> 8 bit equal to 0xa8 (why are we not checking for != 0 here?)
I think you found a mistake in the new sysfs file. I can't recall why
I masked the value the way I did.
> To restore the previous functionality, the "easiest" option might be
> to just return SCU50[31..0] as is, without trying to interpret it.
I think we will go with this option. This remains compatible with the
uart_render_controller code, which is our only known user of the file.
> Another option would be to check whether it's != 0 (instead of & ) -
> I think this would work in my example, too. If that's an option, I'll
> test it.
> For the check in , what BIOS was this tested with? Would a != 0
> check work there?
I would have tested it with a Power9 host, which doesn't run the BIOS
and relies on the "ast" DRM driver to perform the setup. Looking at
that driver it calls ast_open_key which writes 0xa8:
I suspect my testing was bad.
Thanks for investigating this. I'll send a patch to the kernel and if
you could test it that would be appreciated.
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