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    SubjectRe: [PATCH v1 09/47] vidoe: fbdev: atyfb: remove and fix MTRR MMIO "hole" work around
    On Sat, Mar 21, 2015 at 11:15:14AM +0200, Ville Syrjälä wrote:
    > On Fri, Mar 20, 2015 at 04:17:59PM -0700, Luis R. Rodriguez wrote:
    > > diff --git a/drivers/video/fbdev/aty/atyfb_base.c b/drivers/video/fbdev/aty/atyfb_base.c
    > > index 8025624..8875e56 100644
    > > --- a/drivers/video/fbdev/aty/atyfb_base.c
    > > +++ b/drivers/video/fbdev/aty/atyfb_base.c
    > > @@ -2630,21 +2630,10 @@ static int aty_init(struct fb_info *info)
    > >
    > > #ifdef CONFIG_MTRR
    > > par->mtrr_aper = -1;
    > > - par->mtrr_reg = -1;
    > > if (!nomtrr) {
    > > - /* Cover the whole resource. */
    > > - par->mtrr_aper = mtrr_add(par->res_start, par->res_size,
    > > + par->mtrr_aper = mtrr_add(info->fix.smem_start,
    > > + info->fix.smem_len,
    > > MTRR_TYPE_WRCOMB, 1);
    >
    > MTRRs need power of two size, so how is this supposed to work?

    As per mtrr_add_page() [0] the base and size are just supposed to be in units
    of 4 KiB, although the practice is to use powers of 2 in *some* drivers this
    is not standardized and by no means recorded as a requirement. Obviously
    powers of 2 will work too and you'd end up neatly aligned as well. mtrr_add()
    will use mtrr_check() to verify the the same requirement. Furthermore,
    as per my commit log message:

    ---
    The last thing we do must do to remain sane is ensure we
    use the info->fix.smem_start and info->fix.smem_len for
    the framebuffer MTRR as we know that is always well adjusted.
    The *one* concern here would be if the MTRR is not in units
    of 4K __but__ we already know that in the PCI case this cannot
    happen, in the shared space setting the MTRR would be up to
    0x7ff000 and assuming a 4K page:

    ; 0x7ff000 / 0x1000
    2047

    Also, internally when MTRR is used mtrr_add() will use mtrr_check()
    and that should splat a warning when the MTRR base and size are
    not compatible with what is expected for MTRR usage.
    ---

    If any of this is too risky we can use the __arch_phys_wc_add() (or as
    Andy suggested perhaps use set_page_* stuff, although I am still evaluating
    this) but I did this change to show the effort required for a change when
    the registers / framebuffer is on the same PCI BAR but at different offsets.

    [0] scripts/kernel-doc -man -function mtrr_add_page arch/x86/kernel/cpu/mtrr/main.c | nroff -man | less

    Luis


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