Re: [sqcam-devel] [REVIEW] Driver for SQ-905 based cameras

[Hans de Goede <j.w.r.degoede@hhs.nl>]

kilgota@banach.math.auburn.edu wrote:
> 
> 
> On Thu, 1 Jan 2009, Adam Baker wrote:
> 
>> On Thursday 01 January 2009, Hans de Goede wrote:
> 
>>>
>>> "\x0" will point to const memory, this is not allowed as a buffer 
>>> passed to
>>> usb_control_msg, instead you should use a r/w buffer suitable for DMA.
>>> We've got gspca_dev->usb_buf for this.
> 
> Perhaps my ignorance is showing here. If one actually specifies a value 
> to put into that slot, then why does it "point to const memory" and if 
> so why is that a sin? Sorry if you think this is a dumb question, but I 
> am a bit new to doing this kind of device support at the kernel level. 
> In userspace stuff like libgphoto2 such things are obviously of much 
> less significance.
> 

Well for one const memory is read only and usb_control_msg might try to write 
to it. Another problem is that usb_control_msg() needs a dma-able buffer. In 
kernelspace not all memory is equal. In this case the memory needs to match 
certain alignment criteria and might need to be in a certain physical address 
range (ISA for example can not do dma to physical addresses above 16 megabyte, 
32 bit PCI devices cannot do dma above 4 Gigabyte, etc.).

Regards,

Hans

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