I have a product that is basically a USB flash drive based on an NXP LPC18xx microcontroller. I'm using a library provided from the manufacturer (LPCOpen) that handles the USB MSC and the SD card media (which is where I store data).
Here is the problem: Internally the LPC18xx has a 64kB (limited by hardware) buffer used to cache reads/writes which means it can only cache up to 128 blocks(512B) of memory. The SCSI Write-10 command has a total-blocks field that can be up to 256 blocks (128kB). When originally testing the product on Windows 7 it never writes more than 128 blocks at a time but when tested on Linux it sometimes writes more than 128 blocks, which causes the microcontroller to crash.
Is there a way to tell the host OS not to request more than 128 blocks? I see references[1] to a Read-Block-Limit command(05h) but it doesn't seem to be widely supported. Also, what sense key would I return on the Write-10 command to tell Linux the write is too large? I also see references to a block limit VPD page in some device spec sheets but cannot find a lot of documentation about how it is implemented.
Let me offer a disclaimer up front that this is what you SHOULD do, but none of this may work. A cursory search of the Linux SCSI driver didn't show me what I wanted to see. So, I'm not at all sure that "doing the right thing" will get you the results you want.
Going by the book, you've got to do two things: implement the Block Limits VPD and handle too-large transfer sizes in WRITE AND READ.
First, implement the Block Limits VPD page, which you can find in late revisions of SBC-3 floating around on the Internet (like this one: http://www.13thmonkey.org/documentation/SCSI/sbc3r25.pdf). It's probably worth going to the t10.org site, registering, and then downloading the last revision (http://www.t10.org/cgi-bin/ac.pl?t=f&f=sbc3r36.pdf).
The Block Limits VPD page has a maximum transfer length field that specifies the maximum number of blocks that can be transferred by all the READ and WRITE commands, and basically anything else that reads or writes data. Of course the downside of implementing this page is that you have to make sure that all the other fields you return are correct!
Second, when handling READ and WRITE, if the command's transfer length exceeds your maximum, respond with an ILLEGAL REQUEST key, and set the additional sense code to INVALID FIELD IN CDB. This behavior is indicated by a table in the section that describes the Block Limits VPD, but only in late revisions of SBC-3 (I'm looking at 35h).
You might just start with returning INVALID FIELD IN CDB, since it's the easiest course of action. See if that's enough?