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

[Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Nov 13, 2025 at 12:10:16PM +0100, Jan Beulich wrote:
> This way the relatively large chunk of DMA buffers can be freed when the
> driver isn't in use.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- 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). Previously .bss.page_aligned contained (hopefully) only
page-aligned objects, but now brk_alloc() can combine them.

>  
>  static int __init cf_check xhci_parse_dbgp(const char *opt_dbgp)
>  {
> @@ -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.

>                  uart->dbc.sbdf,
>                  "XHCI console");
>          serial_register_uart(SERHND_XHCI, &dbc_uart_driver, &dbc_uart);
> 

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

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