[PATCH] usb: xhci: fix ERST max field conversion

[PATCH] usb: xhci: fix ERST max field conversion

[Youngjoon Lee]

Commit 6d45e9556d4a ("usb: xhci: standardize multi bit-field
macros") converted the ERST maximum calculation from BIT() to a
left shift. These operations are not equivalent because the
HCS_ERST_MAX field contains an exponent.

On an MT8173 xHCI controller where HCS_ERST_MAX is zero, the new
calculation produces zero event ring segments. This reaches
dma_alloc_coherent() with a zero size, triggers a page allocator
warning, and prevents xHCI from probing with -ENOMEM.

Preserve the original calculation by applying BIT() to the value
extracted with FIELD_GET().

This restores the USB 2.0 and USB 3.0 root hubs on MT8173. A USB
webcam and both USB 2.0 and USB 3.0 hubs enumerate successfully,
and the previous ERST allocation failure is no longer present.

Fixes: 6d45e9556d4a ("usb: xhci: standardize multi bit-field macros")
Signed-off-by: Youngjoon Lee <getfeus@gmail.com>
---
 drivers/usb/host/xhci-mem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index 7a21ac81f9c8..af8d4b74c4ba 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -2301,7 +2301,7 @@ xhci_alloc_interrupter(struct xhci_hcd *xhci, unsigned int segs, gfp_t flags)
 	if (!segs)
 		segs = ERST_DEFAULT_SEGS;
 
-	max_segs = FIELD_GET(HCS_ERST_MAX, xhci->hcs_params2) << 2;
+	max_segs = BIT(FIELD_GET(HCS_ERST_MAX, xhci->hcs_params2));
 	segs = min(segs, max_segs);
 
 	ir = kzalloc_node(sizeof(*ir), flags, dev_to_node(dev));
-- 
2.55.0

Re: [PATCH] usb: xhci: fix ERST max field conversion

[Neronin, Niklas]

On 27/08/2026 8.37, Youngjoon Lee wrote:
> Commit 6d45e9556d4a ("usb: xhci: standardize multi bit-field
> macros") converted the ERST maximum calculation from BIT() to a
> left shift. These operations are not equivalent because the
> HCS_ERST_MAX field contains an exponent.
> 
> On an MT8173 xHCI controller where HCS_ERST_MAX is zero, the new
> calculation produces zero event ring segments. This reaches
> dma_alloc_coherent() with a zero size, triggers a page allocator
> warning, and prevents xHCI from probing with -ENOMEM.
> 
> Preserve the original calculation by applying BIT() to the value
> extracted with FIELD_GET().
> 
> This restores the USB 2.0 and USB 3.0 root hubs on MT8173. A USB
> webcam and both USB 2.0 and USB 3.0 hubs enumerate successfully,
> and the previous ERST allocation failure is no longer present.
> 
> Fixes: 6d45e9556d4a ("usb: xhci: standardize multi bit-field macros")
> Signed-off-by: Youngjoon Lee <getfeus@gmail.com>

Thanks for the report and patch.

This issue has already been addressed in:

https://lore.kernel.org/linux-usb/7e6648ad-7459-4248-8c6c-227cfc2586d8@linux.intel.com/

-Niklas