Mike Christie wrote: > Douglas Gilbert wrote: >> Mike, >> I see you are removing the scatter_elem_sz parameter. >> What decides the scatter gather element size? Can it >> be greater than PAGE_SIZE? > > Oh yeah, sorry I should have documented that. > > I just made the code try to allocate as large a element as possible. > So the code looks at q->max_segment_size and tries to allocate segments > that large initially. If that is too large then it will drop down by > half like what sg.c used to do when it could not allocate large segments. > > I will add the param back if you want. I had thought it was a workaound > due to the segment size of a device not being exported. > >> >> *** Generalizing the idea of a mmap-ed reserve buffer to >> something the user had more control over could be very >> powerful. >> For example allowing two file descriptors (to different >> devices) in the same process to share the same mmap-ed >> area. This would allow a device to device copy to DMA into >> and out of the same memory, potentially with large per command >> transfers and with no per command scatter gather build and >> tear down. Basically a zero copy copy with minimal CPU >> overhead. >> > > I was thinking of something similar but not based on mmap. I have been > trying to figure out a way to do sg io splice. I do not care what > interface or method is used, I think it would be useful. > > I know we talked about the mmap approach a little, but I do not remember > if we talked about how to tell both fds that they are going to use the > same buffer. Would we need a modification to the sg header or would we > need to add in a new IOCTL which would tell sg.c to share the buffer > between two fds? Mike, Currently there is a flag in sgv3: #define SG_FLAG_MMAP_IO 4 and when it is active the dxferp field is ignored as it is assumed the user previously did a mmap() call to get the reserved buffer. We could add a: #define SG_FLAG_MMAP_IO_SHARED 8 and then the pointer in dxferp could taken as the already mmap-ed buffer from another device. Having more than one mmap-ed IO buffer per file descriptor would be nice but opening multiple file descriptors to the same device can give the same effect (with perhaps a POSIX thread per file descriptor). Doug Gilbert - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
