On Mon, Sep 14, 2020 at 09:14:44PM -0500, Sid Spry wrote: > Hi, after a fair bit of looking I do not see a way to do a hard reset of > a device. Windows seems to expose some functionality that hints these things > are possible. Does anyone know if it is and is implemented in the Linux > drivers? As Greg mentioned, on many systems it is not possible. On systems where it is possible, it can be performed by running a utility program. > I only find reference to automatic power management in /sys. > > The ioctl in the title (IOCTL_USB_HUB_CYCLE_PORT) is joined by others that seem > to actually control the power to the port. I don't understand this sentence. What do you mean when you say that IOCTL_USB_HUB_CYCLE_PORT is joined by others? And I don't see how there can be any ioctls that control power to a USB port when the hardware itself doesn't permit power to be turned off. > I recently saw patches that would control hub power state via additional > hardware. URLs for these patches? > It would be good to have a uniform way for userspace to take > advantage of this power control. Userspace may have further hints that can be > applied to power consumption, and may need to decide to undertake device > recovery. > > What originally made me investigate this issue was a series of MCU development > boards that crash needing power removed to recover. This seems to be impossible > to do on most hardware. Why? Indeed, it is impossible on a lot of hardware. But how come you are asking us why? You should be asking the hardware designers. > There's no good reason to remove a complete power > down from the spec. There are also instances where a hard reset for PCIe, e.g. > cards malfunctioning when used with VFIO-VGA would have saved hardware from > being hopelessly broken. So if you are involved in hardware design please > consider these things. We are involved in software design, not hardware design. Alan Stern
