Re: [PATCH v4 5/5] usb: host: add xhci-exynos driver

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 28.4.2022 6.03, Jung Daehwan wrote:
> On Wed, Apr 27, 2022 at 07:25:21PM +0300, Mathias Nyman wrote:
>> On 26.4.2022 12.18, Daehwan Jung wrote:
>>> This driver is for Samsung Exynos xhci host conroller. It uses xhci-plat
>>> driver mainly and extends some functions by xhci hooks and overrides.
>>>
>>> It supports USB Audio offload with Co-processor. It only cares DCBAA,
>>> Device Context, Transfer Ring, Event Ring, and ERST. They are allocated
>>> on specific address with xhci hooks. Co-processor could use them directly
>>> without xhci driver after then.
>>>
>>> Signed-off-by: Daehwan Jung <dh10.jung@samsung.com>
>>
>> I have to agree with Krzysztof's comments, this is an odd driver stub.
>>
>> Perhaps open up a bit how the Exynos offloading works so we can figure out
>> in more detail what the hardware needs from software.  
>>
>> (...)
> 
>>> +static int xhci_alloc_segments_for_ring_uram(struct xhci_hcd *xhci,
>>> +		struct xhci_segment **first, struct xhci_segment **last,
>>> +		unsigned int num_segs, unsigned int cycle_state,
>>> +		enum xhci_ring_type type, unsigned int max_packet, gfp_t flags,
>>> +		u32 endpoint_type)
>>> +{
>>> +	struct xhci_segment *prev;
>>> +	bool chain_links = false;
>>> +
>>> +	while (num_segs > 0) {
>>> +		struct xhci_segment *next = NULL;
>>> +
>>> +		if (!next) {
>>> +			prev = *first;
>>> +			while (prev) {
>>> +				next = prev->next;
>>> +				xhci_segment_free(xhci, prev);
>>> +				prev = next;
>>> +			}
>>> +			return -ENOMEM;
>>
>> This always return -ENOMEM
> 
> Yes. it's right to return error here.
> 

Still don't think that is the case.

So if the num_segs value passed to a function named
xhci_alloc_segments_for_ring_uram() is anything else than 0, it will 
automatically return -ENOMEM?

>>
>> Also this whole function never allocates or remaps any memory.
> 
> This fuctions is for link segments. Right below function(xhci_ring_alloc_uram)
> allocates.

Still doesn't allocate any ring segments.
Below function only allocates memory for the
ring structure that contains pointers to segments.

> 
>>
>>> +		}
>>> +		xhci_link_segments(prev, next, type, chain_links);
>>> +
>>> +		prev = next;
>>> +		num_segs--;
>>> +	}
>>> +	xhci_link_segments(prev, *first, type, chain_links);
>>> +	*last = prev;
>>> +
>>> +	return 0;
>>> +}
>>> +
>>> +static struct xhci_ring *xhci_ring_alloc_uram(struct xhci_hcd *xhci,
>>> +		unsigned int num_segs, unsigned int cycle_state,
>>> +		enum xhci_ring_type type, unsigned int max_packet, gfp_t flags,
>>> +		u32 endpoint_type)
>>> +{
>>> +	struct xhci_ring	*ring;
>>> +	int ret;
>>> +	struct device *dev = xhci_to_hcd(xhci)->self.sysdev;
>>> +
>>> +	ring = kzalloc_node(sizeof(*ring), flags, dev_to_node(dev));
>>> +	if (!ring)
>>> +		return NULL;
>>> +
>>> +	ring->num_segs = num_segs;
>>> +	ring->bounce_buf_len = max_packet;
>>> +	INIT_LIST_HEAD(&ring->td_list);
>>> +	ring->type = type;
>>> +	if (num_segs == 0)
>>> +		return ring;
>>> +
>>> +	ret = xhci_alloc_segments_for_ring_uram(xhci, &ring->first_seg,
>>> +			&ring->last_seg, num_segs, cycle_state, type,
>>> +			max_packet, flags, endpoint_type);
>>> +	if (ret)
>>> +		goto fail;
>>> +
>>> +	/* Only event ring does not use link TRB */
>>> +	if (type != TYPE_EVENT) {
>>> +		/* See section 4.9.2.1 and 6.4.4.1 */
>>> +		ring->last_seg->trbs[TRBS_PER_SEGMENT - 1].link.control |=
>>> +			cpu_to_le32(LINK_TOGGLE);
>>
>> No memory was allocated for trbs
> 
> Allcation function for trbs are missed. It's done by ioremap.
> I will add it on next submission. Thanks for the comment.
> 
>>
>> A lot of this code seems to exists just to avoid xhci driver from allocating
>> dma capable memory, we can refactor the existing xhci_mem_init() and move
>> dcbaa and event ring allocation and other code to their own overridable
>> functions.
>>
>> This way we can probably get rid of a lot of the code in this series.
> 
> Yes right. I think it's proper. Do you agree with it or have better way
> to do it?

Could be, but I don't have a good picture of how this Exynos audio offloading
works, so it's hard to guess.

Thanks
-Mathias