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SubjectRe: [PATCH 12/18] soc: qcom: ipa: immediate commands
On Sun, May 12, 2019 at 3:25 AM Alex Elder <elder@linaro.org> wrote:

> +/* Initialize header space in IPA local memory */
> +int ipa_cmd_hdr_init_local(struct ipa *ipa, u32 offset, u32 size)
> +{
> + struct ipa_imm_cmd_hw_hdr_init_local *payload;
> + struct device *dev = &ipa->pdev->dev;
> + dma_addr_t addr;
> + void *virt;
> + u32 flags;
> + u32 max;
> + int ret;
> +
> + /* Note: size *can* be zero in this case */
> + if (size > field_max(IPA_CMD_HDR_INIT_FLAGS_TABLE_SIZE_FMASK))
> + return -EINVAL;
> +
> + max = field_max(IPA_CMD_HDR_INIT_FLAGS_HDR_ADDR_FMASK);
> + if (offset > max || ipa->shared_offset > max - offset)
> + return -EINVAL;
> + offset += ipa->shared_offset;
> +
> + /* A zero-filled buffer of the right size is all that's required */
> + virt = dma_alloc_coherent(dev, size, &addr, GFP_KERNEL);
> + if (!virt)
> + return -ENOMEM;
> +
> + payload = kzalloc(sizeof(*payload), GFP_KERNEL);
> + if (!payload) {
> + ret = -ENOMEM;
> + goto out_dma_free;
> + }
> +
> + payload->hdr_table_addr = addr;
> + flags = u32_encode_bits(size, IPA_CMD_HDR_INIT_FLAGS_TABLE_SIZE_FMASK);
> + flags |= u32_encode_bits(offset, IPA_CMD_HDR_INIT_FLAGS_HDR_ADDR_FMASK);
> + payload->flags = flags;
> +
> + ret = ipa_cmd(ipa, IPA_CMD_HDR_INIT_LOCAL, payload, sizeof(*payload));
> +
> + kfree(payload);
> +out_dma_free:
> + dma_free_coherent(dev, size, virt, addr);
> +
> + return ret;
> +}

This looks rather strange. I think I looked at it before and you explained
it, but I have since forgotten what you do it for, so I assume everyone else
that tries to understand this will have problems too.

The issue I see is that you do an expensive dma_alloc_coherent()
but then never actually use the pointer returned by it, only the
dma address that cannot be turned back into a virtual address
in order to access the data in it.

If you can't actually use payload->hdr_table_addr, why even allocate
it here?

Arnd

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