Re: [PATCH v2 1/1] drm/panfrost: Add support for devcoredump

[Alyssa Rosenzweig <alyssa@collabora.com>]

Hi Adrian,

Great work on the devcoredump support! This is really cool to see coming
along, thank you! I've left a few notes below:

> +		if (panfrost_dump_registers[i] >= JS_HEAD_LO(0) &&
> +		    panfrost_dump_registers[i] <= JS_CONFIG_NEXT(0))
> +			js_as_offset = slot * 0x80;
> +		else if (panfrost_dump_registers[i] >= AS_TRANSTAB_LO(0) &&
> +			 panfrost_dump_registers[i] <= AS_STATUS(0))
> +			js_as_offset = ((as_nr) << 6);

I'm not a fan of the magic numbers. Do you think it makes sense to add

	#define JS_SLOT_STRIDE 0x80
	#define MMU_AS_SHIFT 0x6

in the appropriate places in panfrost_regs.h, reexpress the existing
#defines in terms of those

	#define JS_HEAD_LO(n) (JS_BASE + ((n) * JS_SLOT_STRIDE) + 0x00)
	...
	#define JS_FLUSH_ID_NEXT(n) (JS_BASE + ((n) * JS_SLOT_STRIDE) + 0x70)
	...
	#define MM_AS(as) (0x2400 + ((as) << MMU_AS_SHIFT)

and then use those here?

Also, drop the parans around (as_nr), this isn't a macro.

> +	/* Add in the active buffer objects */
> +	for (i = 0; i < job->bo_count; i++) {
> +		dbo = job->bos[i];
> +		file_size += dbo->size;
> +		n_bomap_pages += dbo->size >> PAGE_SHIFT;
> +		n_obj++;
> +	}

Strictly, I don't think this is right -- what happens if the CPU is
configured to use 16K or 64K pages? -- however, that mistake is pretty
well entrenched in panfrost.ko right now and it doesn't seem to bother
anyone (non-4K pages on arm64 are pretty rare outside of fruit
computers).

That said, out-of-context there looks like an alignment question. Could
we add an assert for that, documenting the invariant:

	WARN_ON(!IS_ALIGNED(dbo->size, PAGE_SIZE));

> +	iter.start = __vmalloc(file_size, GFP_KERNEL | __GFP_NOWARN |
> +			__GFP_NORETRY);
> +	if (!iter.start) {
> +		dev_warn(pfdev->dev, "failed to allocate devcoredump file\n");
> +		return;
> +	}
> ...
> +	memset(iter.hdr, 0, iter.data - iter.start);

Why are we using __GFP_NOWARN and __GFP_NORETRY? Why not plain vmalloc?

Also, why vmalloc instead of vzalloc? (Or adding __GFP_ZERO to the list
of __vmalloc flags if __GFP_NOWARN/__GFP_NORETRY are really needed?) Are
there relevant performance or security considerations?

> +/* Definitions for coredump decoding in user space */
> +#define PANFROSTDUMP_VERSION_1 1

I'm not a fan of an enum that just represents a number. Using the
numbers directly means we can compare them in a natural way. Also, using
a major/minor split like Steven suggested can help with semantic
versioning.

Cheers,
Alyssa