Could USB ever take over PCI Post: 302866215

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Special Forums Hardware Could USB ever take over PCI Post 302866215 by lupin..the..3rd on Monday 21st of October 2013 01:44:50 PM

10-21-2013

Registered User

Quote:

Originally Posted by Lost in Cyberia

Internally speaking, for your motherboard bus. Would it be possible for a usb standard to be used in place of the PCI or PCIe bus that is now nearly standard on all PC's? If not, why wouldn't this work?

It would not work for several reasons.

Firstly USB does not support DMA transfer modes. USB operates only in PIO mode, which is crufty and old and slow. PATA, SATA, Firewire, SCSI all operate in DMA mode, which is the biggest factor for why they are so much faster interfaces for attaching hard drives than USB.

Secondly USB has very high protocol overhead and poor performance with bulk data transfers. For example, USB 2.0 has a raw bitrate of 480 Mbits/s. That is 60 MB/s. Yet anyone who's used a USB hard drive knows that even on the newest fastest computers, real world USB 2.0 transfer rates are barely over 30 MB/s. I.e. USB can barely achieve 50% efficiency. Firewire, SCSI, SATA/PATA, on the other hand, achieve 90%+ of their raw bitrate in real world performance.

Another point worth mentioning is the abysmal bus power provided by USB. Only 500 mW. It's the worst of all external peripheral busses. Firewire has been providing 3x the bus power for many years now as compared with USB. And since you're evaluating it as a replacement for PCI-E, which carries 75w, you can see that you'd need about 150 USB ports to provide the bus power of a single PCI-E slot.

Lastly USB performs very poorly at isochronous transfers. Isochronous transfers are critical for audio and video work, where precise timing is key. This is why all Pro Audio/Video gear has been on Firewire for many years, instead of USB, because even though the USB standard includes isochronous support, it's a hack, and doesn't work very well in the real world.

USB was originally designed as a replacement for the RS232 serial port, PS/2 keyboard and mouse port, and the joystick port. All very low speed serial devices. And USB works quite well for these devices. For high bitrate bulk data transfers, low latency, etc. USB is a real turd.

As for one replacing the other, if anything, its the other way round. Look at Thunderbolt. It's essentially an external PCI-E, and as you might expect, it performs leagues ahead of USB for bandwidth intensive devices like hard disks or VGA displays.

Last edited by lupin..the..3rd; 10-21-2013 at 04:15 PM..

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LEARN ABOUT CENTOS

usb_buffer_map_sg

USB_BUFFER_MAP_SG(9)						   USB Core APIs					      USB_BUFFER_MAP_SG(9)

NAME

       usb_buffer_map_sg - create scatterlist DMA mapping(s) for an endpoint

SYNOPSIS

       int usb_buffer_map_sg(const struct usb_device * dev, int is_in, struct scatterlist * sg, int nents);

ARGUMENTS

       dev
	   device to which the scatterlist will be mapped

       is_in
	   mapping transfer direction

       sg
	   the scatterlist to map

       nents
	   the number of entries in the scatterlist

RETURN

       Either < 0 (indicating no buffers could be mapped), or the number of DMA mapping array entries in the scatterlist.

NOTE

       The caller is responsible for placing the resulting DMA addresses from the scatterlist into URB transfer buffer pointers, and for setting
       the URB_NO_TRANSFER_DMA_MAP transfer flag in each of those URBs.

       Top I/O rates come from queuing URBs, instead of waiting for each one to complete before starting the next I/O. This is particularly easy
       to do with scatterlists. Just allocate and submit one URB for each DMA mapping entry returned, stopping on the first error or when all
       succeed. Better yet, use the usb_sg_*() calls, which do that (and more) for you.

       This call would normally be used when translating scatterlist requests, rather than usb_buffer_map, since on some hardware (with IOMMUs) it
       may be able to coalesce mappings for improved I/O efficiency.

       Reverse the effect of this call with usb_buffer_unmap_sg.

COPYRIGHT

Kernel Hackers Manual 3.10					     June 2014						      USB_BUFFER_MAP_SG(9)