Re: [PATCH 04/19] drm/i915/gt: Add helper for shmem copy to iosys_map

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On Fri, Feb 04, 2022 at 08:15:12PM +0100, Thomas Zimmermann wrote:
Hi

Am 04.02.22 um 18:44 schrieb Lucas De Marchi:
Add a variant of shmem_read() that takes a iosys_map pointer rather
than a plain pointer as argument. It's mostly a copy __shmem_rw() but
adapting the api and removing the write support since there's currently
only need to use iosys_map as destination.

Reworking __shmem_rw() to share the implementation was tempting, but
finding a good balance between reuse and clarity pushed towards a little
code duplication. Since the function is small, just add the similar
function with a copy/paste/adapt approach.

Cc: Matt Roper <matthew.d.roper@xxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Signed-off-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
---
 drivers/gpu/drm/i915/gt/shmem_utils.c | 33 +++++++++++++++++++++++++++
 drivers/gpu/drm/i915/gt/shmem_utils.h |  3 +++
 2 files changed, 36 insertions(+)

diff --git a/drivers/gpu/drm/i915/gt/shmem_utils.c b/drivers/gpu/drm/i915/gt/shmem_utils.c
index 0683b27a3890..764adefdb4be 100644
--- a/drivers/gpu/drm/i915/gt/shmem_utils.c
+++ b/drivers/gpu/drm/i915/gt/shmem_utils.c
@@ -3,6 +3,7 @@
  * Copyright © 2020 Intel Corporation
  */
+#include <linux/iosys-map.h>
 #include <linux/mm.h>
 #include <linux/pagemap.h>
 #include <linux/shmem_fs.h>
@@ -123,6 +124,38 @@ static int __shmem_rw(struct file *file, loff_t off,
 	return 0;
 }

Here's a good example of how to avoid iosys_map_incr() and use the memcpy offset:

+int shmem_read_to_iosys_map(struct file *file, loff_t off,
+			    struct iosys_map *map, size_t len)
+{
+	struct iosys_map map_iter = *map;

Rather replace map_iter with something like

 unsigned long map_off = 0;

+	unsigned long pfn;
+
+	for (pfn = off >> PAGE_SHIFT; len; pfn++) {
+		unsigned int this =
+			min_t(size_t, PAGE_SIZE - offset_in_page(off), len);
+		struct page *page;
+		void *vaddr;
+
+		page = shmem_read_mapping_page_gfp(file->f_mapping, pfn,
+						   GFP_KERNEL);
+		if (IS_ERR(page))
+			return PTR_ERR(page);
+
+		vaddr = kmap(page);
+		iosys_map_memcpy_to(&map_iter, 0, vaddr + offset_in_page(off),
+				    this);

Use map_off to index into map directly.

+		mark_page_accessed(page);
+		kunmap(page);
+		put_page(page);
+
+		len -= this;
+		iosys_map_incr(&map_iter, this);

Raplace iosys_map_incr() with map_off += this;

+		off = 0;

Maybe off += this ?

Wouldn't that be wrong? AFAICS `off` is the offset of the file (source) and we
zero it here so just the first iteration considers it as a page offset -
next iterations are expected to copy whole pages or the remaining of the
buffer.

Encapsulating the dest offset calculation inside iosys_map with an iter
was a way to avoid this kind of bugs in places that need to keep track
of both source and dest.

Anyway, I disagree and commit here. I will change to map_off and think
about the other cases.

There are also some more complex cases in which copying the map
and work with it avoids other bugs that I will have to think about:

	- patch 9 ("drm/i915/guc: Convert engine record to
	  iosys_map"): as you already noticed we delegate its update to
	  other compilation unit

	- patch 11 ("drm/i915/guc: Convert golden context prep to iosys_map"):
	  info_map, which can point either to the real buffer (in system
	  or IO memory) or to a temporary buffer (system memory)

	- patch 17: it needs to write to 2 parts of struct: passing
	  different offsets depending on the call is IMO more error
	  prone than making sure we are working with the right variable.


thanks
Lucas De Marchi


I think this pattern should be applied to all similar code. As you already noted, iosys_map_incr() is problematic.

Best regards
Thomas

+	}
+
+	return 0;
+}
+
 int shmem_read(struct file *file, loff_t off, void *dst, size_t len)
 {
 	return __shmem_rw(file, off, dst, len, false);
diff --git a/drivers/gpu/drm/i915/gt/shmem_utils.h b/drivers/gpu/drm/i915/gt/shmem_utils.h
index c1669170c351..e1784999faee 100644
--- a/drivers/gpu/drm/i915/gt/shmem_utils.h
+++ b/drivers/gpu/drm/i915/gt/shmem_utils.h
@@ -8,6 +8,7 @@
 #include <linux/types.h>
+struct iosys_map;
 struct drm_i915_gem_object;
 struct file;
@@ -17,6 +18,8 @@ struct file *shmem_create_from_object(struct drm_i915_gem_object *obj);
 void *shmem_pin_map(struct file *file);
 void shmem_unpin_map(struct file *file, void *ptr);
+int shmem_read_to_iosys_map(struct file *file, loff_t off,
+			    struct iosys_map *map, size_t len);
 int shmem_read(struct file *file, loff_t off, void *dst, size_t len);
 int shmem_write(struct file *file, loff_t off, void *src, size_t len);

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev






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