Hi Brian, On Fri, Nov 18, 2011 at 4:48 PM, Brian Gix <bgix@xxxxxxxxxxxxxx> wrote: > Hi Anderson, > > On 11/18/2011 12:36 PM, Anderson Lizardo wrote: >> >> Hi Marcel, >> >> On Fri, Nov 18, 2011 at 3:08 PM, Marcel Holtmann<marcel@xxxxxxxxxxxx> >> wrote: >>> >>> I think one of the most important questions that we have to ask >>> ourselves at some point is if we wanna put ATT into the kernel. >>> >>> The potential candidate that forces us to think about this is HID over >>> Low Energy. However I like to see numbers on how the context switches >>> with keeping ATT in userspace will effect our latency. >> >> I still fail to see how ATT handling in kernel would reduce context >> switches. A GATT operation is composed of one or more (possibly many, >> see e.g. discovery procedures) ATT PDUs. >> >> Unless you are proposing GATT on kernel as well? > > I don't think having GATT in the kernel is necessarily the logical > conclusion to an ATT-to-kernel migration. I was actually referring only to the context switch topic. You still need to send the ATT request parameters from userspace, which is not different from building the PDU on userspace (vs. building on kernel). Unless I'm not understanding what Marcel meant with "ATT on kernel" (which is most probably the case). > Remember that the focus of LE is *not* the discovery process. Discovery (of > Devices, Services and Characteristics) is important, but what makes LE "Low > Energy" is what happens *after* pairing and discovery have been completed, > which ideally would happen One Time Only. The discovery procedure was only an example of multiple PDU operation. Take e.g. "read long". If only ATT is on kernel, it still need multiple context switches for the multiple PDUs involved for the GATT operation. > The LE profiles and services are being spec'd so that they can be highly > efficient over the lifetime of the Low Energy device. The Linux side of the > equation will rarely be an actual Low Energy device: It will just know how > to talk to an LE device in such a way as to make that Button-cell battery > driven device last a couple years if possible. > > For that reason MOST day-to-day communication between a Linux based > "Central" device will in fact be single-ATT procedures. If it takes more > energy and context switches to do discovery using many compound procedures, > then we pay that price, because it should only happens Once. ... and for this reason I don't see much gain in reducing context switches if *only* ATT is on kernel. (note: I'm not defending GATT on kernel). > HID devices in particular, will almost certainly NOT use compound GATT > procedures to communicate after the pairing and discovery point in time has > past. Yes, and also I don't see why LE HID is an example of "many context switches". For sure, you will need a bunch of them for setting up the HID device (discovery + pairing), but after that it will "talk" HID with the kernel input subsystem (If I understood the basics of this profile). Regards, -- Anderson Lizardo Instituto Nokia de Tecnologia - INdT Manaus - Brazil -- To unsubscribe from this list: send the line "unsubscribe linux-bluetooth" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
