Messages in this thread | | | Date | Tue, 22 Feb 2022 09:18:18 -0600 | Subject | Re: [PATCH v3 23/25] bus: mhi: ep: Add support for queueing SKBs to the host | From | Alex Elder <> |
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On 2/22/22 8:38 AM, Manivannan Sadhasivam wrote: > On Tue, Feb 15, 2022 at 04:40:29PM -0600, Alex Elder wrote: >> On 2/12/22 12:21 PM, Manivannan Sadhasivam wrote: >>> Add support for queueing SKBs to the host over the transfer ring of the >>> relevant channel. The mhi_ep_queue_skb() API will be used by the client >>> networking drivers to queue the SKBs to the host over MHI bus. >>> >>> The host will add ring elements to the transfer ring periodically for >>> the device and the device will write SKBs to the ring elements. If a >>> single SKB doesn't fit in a ring element (TRE), it will be placed in >>> multiple ring elements and the overflow event will be sent for all ring >>> elements except the last one. For the last ring element, the EOT event >>> will be sent indicating the packet boundary. >>> >>> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org> >> >> I'm a little confused by this, so maybe you can provide >> a better explanation somehow. >> >> -Alex >> >>> --- >>> drivers/bus/mhi/ep/main.c | 102 ++++++++++++++++++++++++++++++++++++++ >>> include/linux/mhi_ep.h | 13 +++++ >>> 2 files changed, 115 insertions(+) >>> >>> diff --git a/drivers/bus/mhi/ep/main.c b/drivers/bus/mhi/ep/main.c >>> index baf383a4857b..e4186b012257 100644 >>> --- a/drivers/bus/mhi/ep/main.c >>> +++ b/drivers/bus/mhi/ep/main.c >>> @@ -488,6 +488,108 @@ int mhi_ep_process_tre_ring(struct mhi_ep_ring *ring, struct mhi_ep_ring_element >>> return 0; >>> } >>> +int mhi_ep_queue_skb(struct mhi_ep_device *mhi_dev, enum dma_data_direction dir, >>> + struct sk_buff *skb, size_t len, enum mhi_flags mflags) >> >> Why are both skb and len supplied? Will an skb be supplied >> without wanting to send all of it? Must len be less than >> skb->len? I'm a little confused about the interface. >> >> Also, the data direction is *out*, right? You'll never >> be queueing a "receive" SKB? >> > > This was done to be compatible with the MHI host API where the host can queue > SKBs in both directions. But I think I should stop doing this.
OK.
>>> +{ >>> + struct mhi_ep_chan *mhi_chan = (dir == DMA_FROM_DEVICE) ? mhi_dev->dl_chan : >>> + mhi_dev->ul_chan; >>> + struct mhi_ep_cntrl *mhi_cntrl = mhi_dev->mhi_cntrl; >>> + struct device *dev = &mhi_chan->mhi_dev->dev; >>> + struct mhi_ep_ring_element *el; >>> + struct mhi_ep_ring *ring; >>> + size_t bytes_to_write; >>> + enum mhi_ev_ccs code; >>> + void *read_from_loc; >>> + u32 buf_remaining; >>> + u64 write_to_loc; >>> + u32 tre_len; >>> + int ret = 0; >>> + >>> + if (dir == DMA_TO_DEVICE) >>> + return -EINVAL; >> >> Can't you just preclude this from happening, or >> know it won't happen by inspection? >> >>> + >>> + buf_remaining = len; >>> + ring = &mhi_cntrl->mhi_chan[mhi_chan->chan].ring; >>> + >>> + mutex_lock(&mhi_chan->lock); >>> + >>> + do { >>> + /* Don't process the transfer ring if the channel is not in RUNNING state */ >>> + if (mhi_chan->state != MHI_CH_STATE_RUNNING) { >>> + dev_err(dev, "Channel not available\n"); >>> + ret = -ENODEV; >>> + goto err_exit; >>> + } >>> + >> >> It would be nice if the caller could know whether there >> was enough room *before* you start transferring things. >> It's probably a lot of work to get to that point though. >> > > No, the caller will do this check but the check is included here so that we > don't run out of buffers when the packet needs to be splitted. > >>> + if (mhi_ep_queue_is_empty(mhi_dev, dir)) { >>> + dev_err(dev, "TRE not available!\n"); >>> + ret = -EINVAL; >>> + goto err_exit; >>> + } >>> + >>> + el = &ring->ring_cache[ring->rd_offset]; >>> + tre_len = MHI_EP_TRE_GET_LEN(el); >>> + if (skb->len > tre_len) { >>> + dev_err(dev, "Buffer size (%d) is too large for TRE (%d)!\n", >>> + skb->len, tre_len); >> >> This means the receive buffer must be big enough to hold >> any incoming SKB. This is *without* checking for the >> CHAIN flag in the TRE, so what you describe in the >> patch description seems not to be true. I.e., multiple >> TREs in a TRD will *not* be consumed if the SKB data >> requires more than what's left in the current TRE. >> > > I think I removed this check for v3 but somehow the change got lost :/
Looking at this now, it's possible I got confused about which direction the data was moving; but I'm not really sure.
From the perspective of the endpoint device, this is the *transmit* function. But when the device is transmitting, it is moving data into the *receive* buffers that the host has allocated and supplied via the transfer ring.
My statement seems to be correct though, with this logic, the host must supply a buffer large enough to receive the entire next SKB, or it will get an error back. I no longer know what happens when this function (mhi_ep_queue_skb()) returns an error--is the skb dropped?
> But anyway, there is no need to check for CHAIN flag while writing to host. > CHAIN flag is only used or even make sense when host writes data to device, so
I'm not sure that's correct, but I don't want to get into that issue here. We can talk about that separately.
> that it knows the packet boundary and could use the CHAIN flag to tell the > device where the boundary lies.
This doesn't sound to me like what the purpose of the CHAIN flag is, but perhaps I'm misunderstanding you. Let's have a quick private chat about this so we don't waste any more e-mail bandwidth.
-Alex
> But when the device writes to host, it already has the pre-queued elements from > host that has no idea where the packet boundary lies. So the host would've set > only EOT on all TREs and expects the device to send OVERFLOW event for TREs that > don't have the complete packet. Then finally, when device sends EOT event, the > host will detect the boundary. > > Thanks, > Mani
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