linux-kernel - Re: [RFC][PATCH 3/5 v2] dma-buf: heaps: Add system heap to dmabuf heaps

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Date:   Fri, 15 Mar 2019 02:06:49 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Liam Mark <lmark@...eaurora.org>,
        Brian Starkey <Brian.Starkey@....com>,
        "Andrew F . Davis" <afd@...com>, Chenbo Feng <fengc@...gle.com>,
        Alistair Strachan <astrachan@...gle.com>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [RFC][PATCH 3/5 v2] dma-buf: heaps: Add system heap to dmabuf
 heaps

> +	i = sg_alloc_table(table, npages, GFP_KERNEL);
> +	if (i)
> +		goto err1;
> +	for_each_sg(table->sgl, sg, table->nents, i) {
> +		struct page *page;
> +
> +		page = alloc_page(GFP_KERNEL);
> +		if (!page)
> +			goto err2;
> +		sg_set_page(sg, page, PAGE_SIZE, 0);
> +	}

Given that one if not the primary intent here is to DMA map the memory
this is a bad idea, as it might require bounce buffering.

What we really want here is something like this:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-noncoherent-allocator.3

And I wonder if the S/G building should also move into common code
instead of being duplicated everywhere.

> +static int system_heap_create(void)
> +{
> +	struct system_heap *sys_heap;
> +
> +	sys_heap = kzalloc(sizeof(*sys_heap), GFP_KERNEL);
> +	if (!sys_heap)
> +		return -ENOMEM;
> +	sys_heap->heap.name = "system_heap";
> +	sys_heap->heap.ops = &system_heap_ops;
> +
> +	dma_heap_add(&sys_heap->heap);

Why is this dynamically allocated?