[PATCH 1/5] x86: fsp: Use mtrr_set_next_var() for graphics memory

[Simon Glass]

Hi Bin,

On Sun, 23 Jul 2023 at 09:50, Bin Meng <bmeng.cn at gmail.com> wrote:
>
> Hi Simon,
>
> On Sun, Jul 23, 2023 at 11:43 AM Simon Glass <sjg at chromium.org> wrote:
> >
> > Hi Bin,
> >
> > On Fri, 21 Jul 2023 at 10:12, Bin Meng <bmeng.cn at gmail.com> wrote:
> > >
> > > At present this uses mtrr_add_request() & mtrr_commit() combination
> > > to program the MTRR for graphics memory. This usage has two major
> > > issues as below:
> > >
> > > - mtrr_commit() will re-initialize all MTRR registers from index 0,
> > >   using the settings previously added by mtrr_add_request() and saved
> > >   in gd->arch.mtrr_req[], which won't cause any issue but is unnecessary
> > > - The way such combination works is based on the assumption that U-Boot
> > >   has full control with MTRR programming (e.g.: U-Boot without any blob
> > >   that does all low-level initialization on its own, or using FSP2 which
> > >   does not touch MTRR), but this is not the case with FSP. FSP programs
> > >   some MTRRs during its execution but U-Boot does not have the settings
> > >   saved in gd->arch.mtrr_req[] and when doing mtrr_commit() it will
> > >   corrupt what was already programmed previously.
> > >
> > > Correct this to use mtrr_set_next_var() instead.
> > >
> > > Signed-off-by: Bin Meng <bmeng.cn at gmail.com>
> > > ---
> > >
> > >  arch/x86/lib/fsp/fsp_graphics.c | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > Thanks for looking into this. The series works fine on link. I suspect
> > it will be find on samus too, but I cannot test right now. Sadly
> > minnowmax is also dead right now but I hope to fix it soon. I don't
> > expect any problems there.
> >
> > However, for coral, this first patch breaks the mtrrs. With master we get:
> >
> > => mtrr
> > CPU 0:
> > Reg Valid Write-type   Base   ||        Mask   ||        Size   ||
> > 0   Y     Back         00000000fef00000 0000007ffff80000 0000000000080000
> > 1   Y     Back         00000000fef80000 0000007ffffc0000 0000000000040000
> > 2   Y     Back         0000000000000000 0000007f80000000 0000000080000000
> > 3   Y     Combine      00000000b0000000 0000007ff0000000 0000000010000000
> > 4   Y     Back         0000000100000000 0000007f80000000 0000000080000000
> > 5   N     Uncacheable  0000000000000000 0000000000000000 0000008000000000
> > 6   N     Uncacheable  0000000000000000 0000000000000000 0000008000000000
> > 7   N     Uncacheable  0000000000000000 0000000000000000 0000008000000000
> > 8   N     Uncacheable  0000000000000000 0000000000000000 0000008000000000
> > 9   N     Uncacheable  0000000000000000 0000000000000000 0000008000000000
> >
> > with this patch on coral we get:
> >
> > => mtrr
> > CPU 0:
> > Reg Valid Write-type   Base   ||        Mask   ||        Size   ||
> > 0   Y     Back         00000000fef00000 0000007ffff80000 0000000000080000
> > 1   Y     Back         00000000fef80000 0000007ffffc0000 0000000000040000
> > 2   Y     Combine      00000000b0000000 0000007ff0000000 0000000010000000
> > 3   N     Uncacheable  0000000000000000 0000000000000000 0000008000000000
> >
> > At present coral expects to handle the MTRRs itself, and it seems that
> > perhaps the APL FSPv2 does not? Do we need a new Kconfig for dealing
> > with FSPv2 perhaps?
>
> I am a little bit confused. The comment in arch/x86/lib/fsp/fsp_dram.c::
> dram_init_banksize() says:
>
>         /*
>          * For FSP1, the system memory and reserved memory used by FSP are
>          * already programmed in the MTRR by FSP. Also it is observed that
>          * FSP on Intel Queensbay platform reports the TSEG memory range
>          * that has the same RES_MEM_RESERVED resource type whose address
>          * is programmed by FSP to be near the top of 4 GiB space, which is
>          * not what we want for DRAM.
>          *
>          * However it seems FSP2's behavior is different. We need to add the
>          * DRAM range in MTRR otherwise the boot process goes very slowly,
>          * which was observed on Chromebook Coral with FSP2.
>          */
>
> So on Coral with FSP2, U-Boot programs the MTTR by itself.
>
> In this dram_init_banksize(), it calls mtrr_add_request() 3 times, 2
> of which should be what you were seeing as 2 and 4 below:
>
> > 2   Y     Back         0000000000000000 0000007f80000000 0000000080000000
> > 3   Y     Combine      00000000b0000000 0000007ff0000000 0000000010000000
> > 4   Y     Back         0000000100000000 0000007f80000000 0000000080000000
>
> The #3 should be the FSP graphics frame buffer. But I failed to
> understand how the FSP graphics programs a MTRR register in between
> the 2 memory regions programmed by dram_init_banksize() on
> u-boot/master, how could that happen?

Remember that the MTRRs are sorted, so the order or mtrr_add_request()
calls does not matter.

>
> On the other hand, with this patch, how could the FSP graphics memory
> programs a MTRR register that should be programmed by
> dram_init_banksize()?

I believe this adds a new mtrr and then commits the result.

Regards,
Simon