杨苏立 Yang Su Li wrote: > is there some hardware I can by to turn my desktop into an USB > device or USB OTG device? Preferably this hardware should use my > desktop's memory to back-up its storage. Not that I know of. > > If the USB OTG host has a Micro-AB connector then yes, this cable is > > needed to allow a USB device to connect to the OTG host. > > I assume another solution would be directly attach an SSD to the > cellphone, in which case USB transmission/SATA-USB conversion is > supposed to be the bottleneck, as I couldn't find any phone/tablets > which support USB 3.0. Do you guys have any comments on the > performance of such a solution? That is a great idea! If an SSD has good enough performance (and in practise, all SSDs have SDRAM buffers) then go for it. Compare SSD reviews and choose an SSD with performance that you like (I am happily using the Samsung 830 series) and use these two cables: http://www.amazon.com/Seagate-FreeAgent-GoFlex-Upgrade-Cable/dp/B003IT6PHM http://www.amazon.com/T-Flash-Adapter-Samsung-GT-i9100-GT-N7000/dp/B005FUNYSA to connect the SSD as a USB storage device to the cellphone. I have good experience with that particular Seagate USB cable. It may be a good idea to look specifically for a USB 3.0 cable, because the USB chipset is designed for higher performance, so it is perhaps more likely to approach theoretical maximum performance when used with a USB 2.0 host. > >> and an USB host to host cable (like this: > >> http://www.amazon.com/Plugable-USB-Easy-Transfer-Cable/dp/B005OTPVMY > > > > Note that this cable is a "double device" cable, where each end > > simulates a storage device, allowing file transfer between two > > computers. The performance of this cable is probably not amazing. > > If I understand it correctly, I could use the phone as a host using > this cable? A phone supporting OTG can be used as host using this cable: http://www.amazon.com/T-Flash-Adapter-Samsung-GT-i9100-GT-N7000/dp/B005FUNYSA Anything that plugs into this cable on the other end is a device. > And its performance will suffer because it will have to > transform every master command into a slave command? The host-to-host easy transfer cable is a device on both sides, which requires some kind of processing inside the cable to simulate a storage device two times, and the cable is so simple and cheap that the simulation will not likely have very high performance. It also means that the maximum performance is determined by some factor involving the *other* device, connected to the other USB host. This method is not good. > The goal is to simulate cellphone storage. So I think 8/10GB to > 32/64GB would be reasonable here. If that's possible, I can settle > for less than 5GB too. And hopefully the solution would be software > based/light weighted, as unfortunately I don't have too much hardware > programming experience. Since you need several GB backend storage as opposed to 200-400 MB it is rather difficult to find simple hardware solutions. > > In theory you can maybe use a Beagle hardware (BeagleBoard-xM, > > BeagleBone, etc.) to create a RAM-based USB storage device. BUT! > > I do not know if the USB hardware in the TI OMAP chip will approach > > maximum theoretical USB throughput in this configuration. It would > > however be a way to test your idea with low hardware cost. > > So if I understand it right, Beagle board is something which could > operate as an USB device, Yes. Different boards can even support USB OTG; so be either host or device. > and which I could also insert large DRAM cards? Unfortunately not, the development boards I've seen all have a fixed amount of DRAM. BeagleBone has 256 MB, BeagleBoard-xM has 512 MB. There are many other similar development boards, but they all have fixed DRAM. > How hard is it to run a Linux on such a borad then? They usually ship with Linux and there are strong communities of Linux users around them. They are made for Linux. But I do not know what the performance is of the gadget hardware - in particular I don't know if it reaches the theoretical maximum peformance of USB. > If I can easily run LInux, then this solution really seem > appealing then. Do you have any recommendation on specific Beagle > board? If you want to try it I suggest choosing one of BeagleBoard-xM and BeagleBone. The latter is lower cost, smaller, more integrated, but is also slower and has half the DRAM. > > For maximum performance I suggest to design special purpose hardware .. > > Significantly more work, but will have great performance. > > As for this solution, how much development effort would you estimate? Maybe 4-6 months for someone with the right background and good "predisposition" - but without experience from the technologies involved. (CPU, USB, DMA, memory bus, FPGA, SDRAM. It is very much like building a complete computer from scratch.) > I could probably devote one week to setup such a thing, but if it will > require more than two weeks' of work, then it probably doesn't worth > it anyway. If you think the SSD method gives sufficient performance then I suggest doing that. It's by far the fastest and simplest solution. //Peter -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
