Dear Doug,
在 2018年05月11日 04:59, Doug Anderson 写道:
Hi,
On Wed, May 9, 2018 at 1:55 AM, wlf <wulf@xxxxxxxxxxxxxx> wrote:
+ } else if (hsotg->params.host_dma) {
Are you sure this is "else if"? Can't you have descriptor DMA enabled
in the controller and still need to do a normal DMA transfer if you
plug in a hub? Seems like this should be just "if".
Sorry, I don't understand the case "have descriptor DMA enabled in the
controller and still need to do a normal DMA transfer". But maybe it
still
has another problem if just use "if" here, because it will create kmem
caches for Slave mode which actually doesn't need aligned DMA buf.
So right now your code looks like:
if (hsotg->params.dma_desc_enable ||
hsotg->params.dma_desc_fs_enable) {
...
...
} else if (hsotg->params.host_dma) {
...
}
I've never played much with "descriptor DMA" on dwc2 because every
time I enabled it (last I tried was years ago) a whole bunch of
peripherals stopped working and I didn't dig (I just left it off).
Thus, I'm no expert on descriptor DMA. ...that being said, my
understanding is that if you enable descriptor DMA in the controller
then it will enable a smarter DMA mode for some of the transfers.
IIRC Descriptor DMA mode specifically can't handle splits. Is my
memory correct there? Presumably that means that when you enable
descriptor DMA in the controller the driver will automatically fall
back to normal Address DMA mode if things get too complicated. When
it falls back to Address DMA mode, presumably dwc2_hcd_init() won't
get called again. That means that any data structures that are needed
for Address DMA need to be allocated in dwc2_hcd_init() even if
descriptor DMA is enabled because we might need to fall back to
Address DMA.
Thus, I'm suggesting changing the code to:
if (hsotg->params.dma_desc_enable ||
hsotg->params.dma_desc_fs_enable) {
...
...
}
if (hsotg->params.host_dma) {
...
}
...as I said, I'm no expert and I could just be confused. If
something I say seems wrong please correct me.
Ah, I got it. Thanks for your detailed explanation. Although I don't know if
it's possible that dwc2 driver automatically fall back to normal Address DMA
mode when desc DMA work abnormally with split, I agree with your suggestion.
I'll fix it in V4 patch.
Hmm, I guess you're right. I did a quick search and I can't find any
evidence of a fallback like this. Maybe I dreamed that. I found some
old comment in the commit history that said:
I think you're right. I find that it's possible to fall back to Address
DMA mode if desc DMA initialization failure. The error case is：in
dwc2_hcd_init(), if it fails to create dwc2 generic desc cache or dwc2
hs isoc desc cache, then it will disable desc DMA and fall back to
Address DMA mode.
/*
* Disable descriptor dma mode by default as the HW can support
* it, but does not support it for SPLIT transactions.
* Disable it for FS devices as well.
*/
...and it looks there's currently nobody using descriptor DMA anyway
(assuming I read the code correctly). It does seem like people were
ever going to turn it on then it would have to be dynamic as I was
dreaming it was...
Yes, the dwc2 desc DMA mode can't support split transaction. So few
people use desc DMA mode for OTG Host mode. BTW, I find that the latest
code enable desc DMA mode by default for the OTG peripheral mode if the
dwc2 hardware support desc DMA.
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