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drivers/usbhost: Compute the event ring segment count at full width.

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The number of event ring segments a controller allows is a power of two
reported as its exponent, and the exponent can reach 15.  Computing
1 << exponent into the uint8_t that holds it wraps to zero on any
controller offering more than 128 segments, and a controller told its
event ring table holds no entries has nowhere to report anything: every
command times out.

Work it out at full width and narrow afterwards.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Justin Hammond <justin@dynam.ac>
This commit is contained in:
Justin Hammond 2026-08-16 17:15:08 +08:00 • committed by Xiang Xiao
parent ecbd1870d3
commit c288d9f9e4
1 changed files with 12 additions and 6 deletions
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18
drivers/usbhost/usbhost_xhci.c
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@ -4611,6 +4611,7 @@ static void xhci_disconnect(FAR struct usbhost_driver_s *drvr,
static int xhci_hw_getparams(FAR struct usbhost_xhci_s *priv)
{
uint32_t regval;
uint32_t erst;
/* Get data form Host Controller Capability 1 Parameters */
@ -4651,16 +4652,21 @@ static int xhci_hw_getparams(FAR struct usbhost_xhci_s *priv)
uinfo("no scratch = %d\n", priv->no_scratch);
priv->no_erst = 1 << XHCI_HCSPARAMS2_ERST(regval);
/* How many event ring segments the controller will allow, which is a
* power of two and can reach 32768, so it is worked out at full width
* and only then narrowed to what this driver actually uses. Computed
* into the field directly it would wrap to zero on any controller
* offering more than 128 segments, and a table declared to hold no
* entries gives a controller with nowhere to report anything.
*/
uinfo("no_erst = %d\n", priv->no_erst);
erst = 1ul << XHCI_HCSPARAMS2_ERST(regval);
uinfo("erst max = %" PRIu32 "\n", erst);
/* Limit event ring segment table to 1 */
if (priv->no_erst > XHCI_MAX_ERST)
{
priv->no_erst = XHCI_MAX_ERST;
}
priv->no_erst = (erst > XHCI_MAX_ERST) ? XHCI_MAX_ERST : erst;
uinfo("no erst = %d\n", priv->no_erst);