From: Jay Chen shawn2000100@gmail.com
[ Upstream commit f72b9aa821a2bfe4b6dfec4be19f264d0673b008 ]
There is a subtle contradiction between sections of the xHCI 1.2 spec regarding the initialization of Input Endpoint Context fields. Section 4.8.2 ("Endpoint Context Initialization") states that all fields should be initialized to 0. However, Section 6.2.3 ("Endpoint Context", p.453) specifies that the Average TRB Length (avg_trb_len) field shall be greater than 0, and explicitly notes (p.454): "Software shall set Average TRB Length to '8' for control endpoints."
Strictly setting all fields to 0 during initialization conflicts with the specific recommendation for control endpoints. In practice, setting avg_trb_len = 0 is not meaningful for the hardware/firmware, as the value is used for bandwidth calculation.
Motivation: Our company is developing a custom Virtual xHC hardware platform that strictly follows the xHCI spec and its recommendations. During validation, we observed that enumeration fails and a parameter error (TRB Completion Code = 5) is reported if avg_trb_len for EP0 is not set to 8 as recommended by Section 6.2.3. This demonstrates the importance of assigning a meaningful, non-zero value to avg_trb_len, even in virtualized or emulated environments.
This patch explicitly sets avg_trb_len to 8 for EP0 in xhci_setup_addressable_virt_dev(), as recommended in Section 6.2.3, to prevent potential issues with xHCI host controllers that enforce the spec strictly.
Link: https://bugzilla.kernel.org/show_bug.cgi?id=220033 Signed-off-by: Jay Chen shawn2000100@gmail.com Signed-off-by: Mathias Nyman mathias.nyman@linux.intel.com Link: https://lore.kernel.org/r/20250717073107.488599-4-mathias.nyman@linux.intel.... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
Based on my analysis of the commit and code, here's my assessment:
**Backport Status: YES**
This commit should be backported to stable kernel trees for the following reasons:
1. **Fixes a real bug**: The commit addresses a specification compliance issue where setting `avg_trb_len = 0` for EP0 (control endpoint) violates the xHCI 1.2 specification. Section 6.2.3 explicitly states that Average TRB Length shall be greater than 0 and specifically recommends setting it to 8 for control endpoints.
2. **Small and contained fix**: The change is minimal - just adding a single line: ```c ep0_ctx->tx_info = cpu_to_le32(EP_AVG_TRB_LENGTH(8)); ``` This sets the average TRB length field to 8 as recommended by the spec.
3. **Prevents hardware failures**: The commit message indicates this causes actual enumeration failures with parameter errors (TRB Completion Code = 5) on hardware that strictly follows the xHCI specification. This means real devices can fail to enumerate without this fix.
4. **No architectural changes**: This is a simple initialization fix that doesn't change any architectural aspects of the driver. It only ensures proper initialization of a field that was previously left at 0.
5. **Low regression risk**: Setting avg_trb_len to 8 for control endpoints follows the xHCI specification recommendation. Existing hardware that doesn't strictly check this value will continue to work, while hardware that does enforce the spec will now work correctly.
6. **Clear bug with clear fix**: The contradiction between spec sections is well-documented in the commit message, and the fix directly addresses this by following the more specific recommendation for control endpoints.
The commit fixes a specification compliance bug that can cause real hardware failures during USB device enumeration. The fix is minimal, follows the xHCI specification, and has very low risk of causing regressions, making it an ideal candidate for stable backporting.
drivers/usb/host/xhci-mem.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c index 91178b8dbbf0..1111650757ea 100644 --- a/drivers/usb/host/xhci-mem.c +++ b/drivers/usb/host/xhci-mem.c @@ -1172,6 +1172,8 @@ int xhci_setup_addressable_virt_dev(struct xhci_hcd *xhci, struct usb_device *ud ep0_ctx->deq = cpu_to_le64(dev->eps[0].ring->first_seg->dma | dev->eps[0].ring->cycle_state);
+ ep0_ctx->tx_info = cpu_to_le32(EP_AVG_TRB_LENGTH(8)); + trace_xhci_setup_addressable_virt_device(dev);
/* Steps 7 and 8 were done in xhci_alloc_virt_device() */