Re: [PATCH] drm/bochs: use ioremap_wc() to map framebuffer during driver probing

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Hi

Am 09.09.24 um 07:15 schrieb Yan Zhao:
Use ioremap_wc() instead of ioremap() to map framebuffer during driver
probing phase.

Using ioremap() results in a VA being mapped with PAT=UC-. Additionally,
on x86 architectures, ioremap() invokes memtype_reserve() to reserve the
memory type as UC- for the physical range. This reservation can cause
subsequent calls to ioremap_wc() to fail to map the VA with PAT=WC to the
same physical range for framebuffre in ttm_kmap_iter_linear_io_init().
Consequently, the operation drm_gem_vram_bo_driver_move() ->
ttm_bo_move_memcpy() -> ttm_move_memcpy() becomes significantly slow on
platforms where UC memory access is slow.

I've noticed this too and pushed a major update that replaces the entire memory management. [1]

The patch is still welcome, I think, but you may want to rebase onto the latest drm-misc-next branch. [2]

Best regards
Thomas

[1] https://patchwork.freedesktop.org/series/138086/
[2] https://gitlab.freedesktop.org/drm/misc/kernel/-/tree/drm-misc-next


Here's the performance data measured in a guest on the physical machine
"Sapphire Rapids XCC".
With host KVM honors guest PAT memory types, the effective memory type
for this framebuffer range is
- WC when ioremap_wc() is used in driver probing phase
- UC- when ioremap() is used.

The data presented is an average from 10 execution runs.
The memcpy range for the data is
mem->bus.offset=0xfd000000, mem->size=0x3e8000.

--------------------------------------------------------------
                               |      in bochs_hw_init()       |
                               |    ioremap()   | ioremap_wc() |
------------------------------|----------------|--------------|
     cycles of                 |    2227.4M     |   17.8M      |
drm_gem_vram_bo_driver_move() |                |              |
------------------------------|----------------|--------------|
     time of                   |    1.24s       |   0.01s      |
drm_gem_vram_bo_driver_move() |                |              |
--------------------------------------------------------------

Reported-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/87jzfutmfc.fsf@xxxxxxxxxx/#t
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
  drivers/gpu/drm/tiny/bochs.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/tiny/bochs.c b/drivers/gpu/drm/tiny/bochs.c
index 31fc5d839e10..6414f0a72f6a 100644
--- a/drivers/gpu/drm/tiny/bochs.c
+++ b/drivers/gpu/drm/tiny/bochs.c
@@ -261,7 +261,7 @@ static int bochs_hw_init(struct drm_device *dev)
  	if (pci_request_region(pdev, 0, "bochs-drm") != 0)
  		DRM_WARN("Cannot request framebuffer, boot fb still active?\n");
- bochs->fb_map = ioremap(addr, size);
+	bochs->fb_map = ioremap_wc(addr, size);
  	if (bochs->fb_map == NULL) {
  		DRM_ERROR("Cannot map framebuffer\n");
  		return -ENOMEM;

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)




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