On Wed, 25 Jun 2014, Stalley, Sean wrote: > > -----Original Message----- > > From: Alan Stern [mailto:stern@xxxxxxxxxxxxxxxxxxx] > > Sent: Wednesday, June 25, 2014 11:46 AM > > To: Stalley, Sean > > Cc: linux-usb@xxxxxxxxxxxxxxx; Ismail, Abdul R > > Subject: Re: [PATCH] Change wMaxPacketSize default value for Full Speed EP0 > > > > On Wed, 25 Jun 2014, Sean O. Stalley wrote: > > > > > From: "Sean O. Stalley" <sean.stalley@xxxxxxxxx> > > > > > > Change the default wMaxPacketSize value for EP0 of Full Speed devices > > > from the maximum possible size (64) to the minimum (8). > > > All full speed devices support a packet size of 8, but not all support > > > a size of 64. > > > > You don't seem to understand what you are saying. Supporting a packet size of > > 8 is very different from having a maxpacket size of 8. > > I suppose I could have phrased that better. What I meant to say was: I do not understand why one would make the default wMaxPacketSize for EP0 of a full speed device 64. > To me, making the default 8 makes more sense. Neither 8 nor 64 is the default. 64 is merely the initial value; it gets updated later when we read the correct value from the device. 8 won't work with the "new" initialization scheme. Consider what the scheme does: It sends a Get-Device-Descriptor request with a transfer length of 64. Since the descriptor is only 18 bytes long, the device will try to send back an 18-byte reply. If the actual maxpacket size is 64, the device will do this by sending the entire reply in a single 18-byte packet. But if we set the initial maxpacket value in the host controller to 8, the reply would fail with a Babble error because 18 > 8. > > > No packet longer than 8 bytes is ever read from EP0 before this value > > > is changed from the default. > > > > That is not true. > > You are right, that statement is only true when using the old scheme. > Would setting wMaxPacketSize conditionally (based on using the old/new scheme) be preferred? Well, it would work. But it wouldn't be preferred. Why go to the extra trouble? It's easier to set the initial value to 64 always. Alan Stern -- 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
