Re: [PATCH v3 01/10] drivers/char: Add support for USB3 DbC debugger

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Aug 05, 2022 at 11:54:06AM +0200, Jan Beulich wrote:
> On 05.08.2022 11:51, Marek Marczykowski-Górecki wrote:
> > On Fri, Aug 05, 2022 at 09:23:32AM +0200, Jan Beulich wrote:
> >> On 26.07.2022 05:23, Marek Marczykowski-Górecki wrote:
> >>> +static struct xhci_trb evt_trb[DBC_TRB_RING_CAP];
> >>> +static struct xhci_trb out_trb[DBC_TRB_RING_CAP];
> >>> +static struct xhci_trb in_trb[DBC_TRB_RING_CAP];
> >>> +static struct xhci_erst_segment erst __aligned(64);
> >>> +static struct xhci_dbc_ctx ctx __aligned(64);
> >>> +static uint8_t out_wrk_buf[DBC_WORK_RING_CAP] __aligned(DBC_PAGE_SIZE);
> >>
> >> I've been trying to identify the reason for the alignment here,
> >> compared to the other buffers which are no longer page-aligned. I
> >> haven't even been able to locate the place where the address of
> >> this buffer is actually written to hardware; all I could find was
> >> the respective virt_to_maddr(). Could you please point me at that?
> > 
> > It's dbc_flush() -> dbc_push_trb().
> > And indeed, I think I can drop the alignment when it's moved into
> > structure dedicated for DMA-accessible buffers.
> 
> Why would you be able to drop the alignment then, but not here?

Similar reason as previously - to guarantee it isn't put with unrelated
stuff on the same page, as this array will be accessed by the controller
directly. But, since this patch doesn't attempt to handle IOMMU
situation yet, I think I can drop it here too as there is little benefit
for trying here.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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