RAM optimization for USB CDC (2) about stm32cubef4

[ALABSTM]

Related to issue STM32CubeF4#10.

[ALABSTM]

Hi @jrahlf,

Thank you for this other optimization proposal. It will be logged into our internal database too. Identified descriptors that could be placed in read-only region are listed below.

With regards,

STM32CubeF4/Middlewares/ST/STM32_USB_Device_Library/Class/CDC/Src/usbd_cdc.c

Line 116 in 2d5b78e

__ALIGN_BEGIN static uint8_t USBD_CDC_DeviceQualifierDesc[USB_LEN_DEV_QUALIFIER_DESC] __ALIGN_END =

STM32CubeF4/Middlewares/ST/STM32_USB_Device_Library/Class/CDC/Src/usbd_cdc.c

Line 159 in 2d5b78e

__ALIGN_BEGIN static uint8_t USBD_CDC_CfgHSDesc[USB_CDC_CONFIG_DESC_SIZ] __ALIGN_END =

STM32CubeF4/Middlewares/ST/STM32_USB_Device_Library/Class/CDC/Src/usbd_cdc.c

Line 255 in 2d5b78e

__ALIGN_BEGIN static uint8_t USBD_CDC_CfgFSDesc[USB_CDC_CONFIG_DESC_SIZ] __ALIGN_END =

STM32CubeF4/Middlewares/ST/STM32_USB_Device_Library/Class/CDC/Src/usbd_cdc.c

Line 349 in 2d5b78e

__ALIGN_BEGIN static uint8_t USBD_CDC_OtherSpeedCfgDesc[USB_CDC_CONFIG_DESC_SIZ] __ALIGN_END =

[ALABSTM]

ST Internal Reference: 99842

[ALABSTM]

Hi @jrahlf,

Regarding this question below, may I ask you to clarify which "documentation" you are referring to?

3. The documentation for the different fields in the descriptors is different for "high speed" / "full speed" / "other speed". Please consolidate that.

Thank you,

[jrahlf]

Hi @ALABSTM , please see the attached image regarding the differences in the documentation. I think only the "OtherSpeed" descriptor is affected. E.g. one documentation says 100mA and the other one says 0mA even though the value is the same. There also other subtle differences, .e.g differences in whitespace or
/* bInterval: ignore for Bulk transfer */ vs
/* bInterval */.

As I now understand, the OtherSpeed desciptor is to inform the host about the possible Highspeed configuration, if the device is running in FullSpeed mode. If that is the case, the wMaxPacketSize field should be set to 512 instead of 64 (for the OtherSpeed descriptor). Can you check this?

[ALABSTM]

ST Internal Reference: 99959

[ALABSTM]

Hi @jrahlf,

Thank you for the clarification. The point has been logged into our internal database. Regarding your other question below, I let @CHAMSTM comment.

2. Why is there even a descriptor for "other speed" ? It sets the bDescriptorType field to 7, I cannot find any documentation for this value on the internet. Why is it not 0x2?

With regards,

[ALABSTM]

Hi @jrahlf,

I hope you are doing well. According to our development teams, a couple of use-cases have been identified where the user could modify some fields of the device configuration descriptor at run-time preventing from moving its location into read-only memory.

With regards,

[ALABSTM]

Hi @jrahlf,

I hope you are doing well.

• Regarding moving the device configuration descriptor to a "read-only" memory region, this is not possible according to our development teams. Indeed, the descriptor is stored in pdev->pConfDesc and, depending on the use-case, some fields could be changed at run time. With a simple software disconnect-then-connect operation, the new descriptor will be shared to the host.
• Regarding the documentation found in the comments, it has been updated in the frame of v2.8.0 of the USB Device Library found in the stm32_mw_usb_device repository.

Thank you again for having shared this proposal. Please allow me to close this issue now. Thank you for your comprehension.

Regards,
Aymen