Re: [PATCH] drm/i915: Remove temporary allocation of dma addresses when rotating

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 21/02/2017 15:01, Joonas Lahtinen wrote:
> On pe, 2017-02-17 at 15:10 +0000, Chris Wilson wrote:
>> The object already stores (computed on the fly) the index to dma address
>> so use it instead of reallocating a large temporary array every time we
>> bind a rotated framebuffer.
>>
>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: Matthew Auld <matthew.william.auld@gmail.com>
>> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>
> <SNIP>
>
>> +rotate_pages(struct drm_i915_gem_object *obj,
>> +	     const struct intel_rotation_plane_info *p,
>>  	     struct sg_table *st, struct scatterlist *sg)
>>  {
>>  	unsigned int column, row;
>> -	unsigned int src_idx;
>>
>> -	for (column = 0; column < width; column++) {
>> -		src_idx = stride * (height - 1) + column;
>> -		for (row = 0; row < height; row++) {
>> -			st->nents++;
>> +	for (column = 0; column < p->width; column++) {
>> +		unsigned long src_idx =
>> +			p->stride * (p->height - 1) + column + p->offset;
>> +		for (row = 0; row < p->height; row++) {
>> +			struct scatterlist *src;
>> +			unsigned int n;
>> +
>> +			src = i915_gem_object_get_sg(obj, src_idx, &n);
>
> i915_gem_object_get_sg has variable names obj, n, *offset, so I'd be
> little concerned of sidetracking reader. Rename n into offset?
>
>> +			src_idx -= p->stride;
>> +
>>  			/* We don't need the pages, but need to initialize
>>  			 * the entries so the sg list can be happily traversed.
>>  			 * The only thing we need are DMA addresses.
>>  			 */
>>  			sg_set_page(sg, NULL, PAGE_SIZE, 0);
>> -			sg_dma_address(sg) = in[offset + src_idx];
>> +			sg_dma_address(sg) = sg_dma_address(src) + n*PAGE_SIZE;
>>  			sg_dma_len(sg) = PAGE_SIZE;
>> -			sg = sg_next(sg);
>> -			src_idx -= stride;
>
> I'm not sure why moving this line, might as well hoist all these to the
> for() line.
>
>> +			sg = __sg_next(sg);
>> +
>> +			st->nents++;
>>  		}
>>  	}
>>
>> @@ -3074,62 +3079,30 @@ static noinline struct sg_table *
>>  intel_rotate_pages(struct intel_rotation_info *rot_info,
>>  		   struct drm_i915_gem_object *obj)
>>  {
>> -	const unsigned long n_pages = obj->base.size / PAGE_SIZE;
>> -	unsigned int size = intel_rotation_info_size(rot_info);
>> -	struct sgt_iter sgt_iter;
>> -	dma_addr_t dma_addr;
>> -	unsigned long i;
>> -	dma_addr_t *page_addr_list;
>> -	struct sg_table *st;
>> +	const unsigned int size = intel_rotation_info_size(rot_info);
>
> This is only used once, just inline it.
>
> Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
>
> Could use an A-b from Tvrtko.

I did not like it in another thread, well, better say I was concerned 
about the increased memory use by the radix tree which would then stick 
around for the obj->pages lifetime (long time for a framebuffer I 
thought). While the temporary array allocations here are not that big 
and very temporary.

I guess someone needs to bite the bullet and try and figure out how 
exactly big is the radix tree for some mixes of more or less coalesced 
sg entries.

Regards,

Tvrtko
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