Re: [PATCH 3/3] xhci-dbc: use brk_alloc()

[Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Nov 13, 2025 at 01:50:28PM +0100, Jan Beulich wrote:
> On 13.11.2025 13:39, Marek Marczykowski wrote:
> > On Thu, Nov 13, 2025 at 12:10:16PM +0100, Jan Beulich wrote:
> >> --- a/xen/drivers/char/xhci-dbc.c
> >> +++ b/xen/drivers/char/xhci-dbc.c
> >> @@ -27,6 +27,8 @@
> >>  #include <xen/serial.h>
> >>  #include <xen/timer.h>
> >>  #include <xen/types.h>
> >> +
> >> +#include <asm/brk.h>
> >>  #include <asm/fixmap.h>
> >>  #include <asm/io.h>
> >>  #include <asm/string.h>
> >> @@ -1321,7 +1323,7 @@ static struct uart_driver dbc_uart_drive
> >>  };
> >>  
> >>  /* Those are accessed via DMA. */
> >> -struct dbc_dma_bufs {
> >> +struct __aligned(PAGE_SIZE) dbc_dma_bufs {
> >>      struct xhci_trb evt_trb[DBC_TRB_RING_CAP];
> >>      struct xhci_trb out_trb[DBC_TRB_RING_CAP];
> >>      struct xhci_trb in_trb[DBC_TRB_RING_CAP];
> >> @@ -1335,8 +1337,7 @@ struct dbc_dma_bufs {
> >>       * DMA-reachable by the USB controller.
> >>       */
> >>  };
> >> -static struct dbc_dma_bufs __section(".bss.page_aligned") 
> >> __aligned(PAGE_SIZE)
> >> -    dbc_dma_bufs;
> >> +DEFINE_BRK(xhci, sizeof(struct dbc_dma_bufs));
> > 
> > I think with this change (or rather with using brk_alloc() below), the
> > structure wants to be padded up to the page size, to avoid putting
> > anything else on the same page (see comment just outside of context
> > above).
> 
> Are you sure? My understanding is that sizeof(xyz) is always evenly divisible 
> by
> alignof(xyz). Hence such padding doesn't need making explicit. (And yes, I did
> see that comment while making the change.)

Ok, then indeed the added (or rather moved) __aligned(PAGE_SIZE) is enough.

Reviewed-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

> >> @@ -1413,24 +1414,33 @@ void __init xhci_dbc_uart_init(void)
> >>  {
> >>      struct dbc_uart *uart = &dbc_uart;
> >>      struct dbc *dbc = &uart->dbc;
> >> +    struct dbc_dma_bufs *dma_bufs;
> >>  
> >>      if ( !dbc->enable )
> >>          return;
> >>  
> >> -    dbc->dbc_ctx = &dbc_dma_bufs.ctx;
> >> -    dbc->dbc_erst = &dbc_dma_bufs.erst;
> >> -    dbc->dbc_ering.trb = dbc_dma_bufs.evt_trb;
> >> -    dbc->dbc_oring.trb = dbc_dma_bufs.out_trb;
> >> -    dbc->dbc_iring.trb = dbc_dma_bufs.in_trb;
> >> -    dbc->dbc_owork.buf = dbc_dma_bufs.out_wrk_buf;
> >> -    dbc->dbc_iwork.buf = dbc_dma_bufs.in_wrk_buf;
> >> -    dbc->dbc_str = dbc_dma_bufs.str_buf;
> >> +    dma_bufs = brk_alloc(sizeof(*dma_bufs));
> >> +    if ( !dma_bufs )
> >> +    {
> >> +        dbc->enable = false;
> >> +        printk(XENLOG_ERR "XHCI: not enough BRK space available\n");
> >> +        return;
> >> +    }
> >> +
> >> +    dbc->dbc_ctx = &dma_bufs->ctx;
> >> +    dbc->dbc_erst = &dma_bufs->erst;
> >> +    dbc->dbc_ering.trb = dma_bufs->evt_trb;
> >> +    dbc->dbc_oring.trb = dma_bufs->out_trb;
> >> +    dbc->dbc_iring.trb = dma_bufs->in_trb;
> >> +    dbc->dbc_owork.buf = dma_bufs->out_wrk_buf;
> >> +    dbc->dbc_iwork.buf = dma_bufs->in_wrk_buf;
> >> +    dbc->dbc_str = dma_bufs->str_buf;
> >>  
> >>      if ( dbc_open(dbc) )
> >>      {
> >>          iommu_add_extra_reserved_device_memory(
> >> -                PFN_DOWN(virt_to_maddr(&dbc_dma_bufs)),
> >> -                PFN_UP(sizeof(dbc_dma_bufs)),
> >> +                PFN_DOWN(virt_to_maddr(dma_bufs)),
> >> +                PFN_DOWN(sizeof(*dma_bufs)),
> > 
> > Is that really correct? But with padding (see earlier comment) it
> > shouldn't really matter.
> 
> I think this is addressed by the reply further up as well.
> 
> Jan

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

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