On Sat, Mar 10, 2018 at 8:57 AM, French, Nicholas A. <naf@xxxxxx> wrote: > On Wed, Mar 07, 2018 at 11:23:09PM -0600, French, Nicholas A. wrote: >> On Thu, Mar 08, 2018 at 04:14:11AM +0000, Luis R. Rodriguez wrote: >> > On Thu, Mar 08, 2018 at 04:06:01AM +0000, Luis R. Rodriguez wrote: >> > > On Thu, Mar 08, 2018 at 03:16:29AM +0000, French, Nicholas A. wrote: >> > > > >> > > > Ah, I see. So my proposed ioremap_wc call was only "working" by aliasing the >> > > > ioremap_nocache()'d mem area and not actually using write combining at all. >> > > >> > > There are some debugging PAT toys out there I think but I haven't played with >> > > them yet or I forgot how to to confirm or deny this sort of effort, but >> > > likeley. >> > >> > In fact come to think of it I believe some neurons are telling me that if >> > two type does not match we'd get an error? > > I can confirm that my original suggested patch just aliases to ivtv-driver's nocache mapping: > $ sudo modprobe ivtvfb > $ sudo dmesg > ... > x86/PAT: Overlap at 0xd5000000-0xd5800000 > x86/PAT: reserve_memtype added [mem 0xd5510000-0xd56b0fff], track uncached-minus, req write-combining, ret uncached-minus > ivtvfb0: Framebuffer at 0xd5510000, mapped to 0x00000000c6a7ed52, size 1665k > ... > $ sudo cat /sys/kernel/debug/x86/pat_memtype_list | grep 0xd5 > uncached-minus @ 0xd5000000-0xd5800000 > uncached-minus @ 0xd5510000-0xd56b1000 > > So nix that. > >> > No what if the framebuffer driver is just requested as a secondary step >> > after firmware loading? >> >> Its a possibility. The decoder firmware gets loaded at the beginning of the decoder >> memory range and we know its length, so its possible to ioremap_nocache enough >> room for the firmware only on init and then ioremap the remaining non-firmware >> decoder memory areas appropriately after the firmware load succeeds... > > I looked in more detail, and this would be "hard" due to the way the rest of the > decoder offsets are determined by either making firmware calls or scanning the > decoder memory range for magic bytes and other mess. > > I think some smart guy named mcgrof apparently came to the same conclusion > in a really old email chain I found [https://lists.gt.net/linux/kernel/2387536]: > "The ivtv case is the *worst* example we can expect where the firmware > hides from us the exact ranges for write-combining, that we should somehow > just hope no one will ever do again." > :-) This is tribal knowledge worth formalizing a bit more for the long run for this ivtv driver. >> Perhaps the easy answer is to change the fatal is-pat-enabled check to just a >> warning like "you have PAT enabled, so wc is disabled for the framebuffer. >> if you want wc, use the nopat parameter"? > > I like this idea more and more. I haven't experience any problems running > with PAT-enabled and no write-combining on the framebuffer. Any objections? I think its worth it, and perhaps best folded under a new kernel parameter option which also documents the limitation noted above, thereby knocking two birds with one stone. This way also users who *want* to opt-in to PAT do so willing-fully and knowing of the limitation. The kconfig option can just enable a module parameter to a default value, which if the kconfig is disabled would otherwise be unset. static bool ivtv_force_pat = IS_ENABLED(CONFIG_IVTV_WHATEVER); module_param_named(force_pat, ivtv_force_pat, bool, S_IRUGO | S_IWUSR); Luis
