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USB: clarify USB_DT_USB_SSP_CAP_SIZE(ssac) definition

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USB_DT_USB_SSP_CAP_SIZE(ssac) gives the size of the SSP capability
descriptor. The descriptor consists of 12 bytes plus a array of
SSA entries.

The number of SSA entries is stored in a SSAC value in the first 12 bytes,
The USB3.1 specification 9.6.2.5 defines SSAC as zero based:
"The number of Sublink Speed Attributes = SSAC + 1." This is not
intuitive and has already caused some confusion.

Make a small modifiaction to the USB_DT_USB_SSP_CAP_SIZE(ssac)
definition to make it a bit clearer

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Mathias Nyman 2017-12-19 13:28:30 +02:00 committed by Greg Kroah-Hartman
parent e142dc1ecc
commit 7c0143153c
1 changed files with 2 additions and 2 deletions
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4
include/uapi/linux/usb/ch9.h
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@ -1077,9 +1077,9 @@ struct usb_ptm_cap_descriptor {
#define USB_DT_USB_PTM_ID_SIZE 3
/*
* The size of the descriptor for the Sublink Speed Attribute Count
* (SSAC) specified in bmAttributes[4:0].
* (SSAC) specified in bmAttributes[4:0]. SSAC is zero-based
*/
#define USB_DT_USB_SSP_CAP_SIZE(ssac) (16 + ssac * 4)
#define USB_DT_USB_SSP_CAP_SIZE(ssac) (12 + (ssac + 1) * 4)
/*-------------------------------------------------------------------------*/