Laurent, To follow up with some of the comments I made before I got additional clarification about the commands supported by the proxy driver running on the VPSS MCU. The proxy will support all of the commands used by V4L2 as well as those proposed extensions to V4L2 that Hans has mentioned. Basically, the list of commands supported at initial release is not only those required today, but a full set for all the features of the VPSS. In this was as new APIs are added to V4L2 the support for those features will already be supported by the VPSS MCU proxy driver. As for the license of the firmware this is still being worked. It is currently under TI proprietary license and will be distributed as binary under Technical Software Publicly Available (TSPA) which means it can be obtained by anyone. If you feel that source code is required for the firmware at launch to gain acceptance please let us know and we can start working that issue. Sincerely, Chase Maupin Software Applications Catalog DSP Products e-mail: chase.maupin@xxxxxx phone: (281) 274-3285 For support: Forums - http://community.ti.com/forums/ Wiki - http://wiki.davincidsp.com/ > -----Original Message----- > From: Laurent Pinchart [mailto:laurent.pinchart@xxxxxxxxxxxxxxxx] > Sent: Thursday, February 11, 2010 7:23 PM > To: Maupin, Chase > Cc: Hans Verkuil; sakari.ailus@xxxxxxxxxxxxxxxxxxxxxxxxxx; > mchehab@xxxxxxxxxxxxx; vpss_driver_design@xxxxxxxxxxx - This list is to > discuss the VPSS driver design (May contain non-TIers); linux- > media@xxxxxxxxxxxxxxx > Subject: Re: Requested feedback on V4L2 driver design > > Hi Chase, > > On Monday 08 February 2010 16:08:37 Maupin, Chase wrote: > > All, > > > > Texas Instruments (TI) is working on the design for the V4L2 capture and > > display drivers for our next generation system-on-chip (SoC) processor > and > > would like to solicit your feedback. > > Thank you very much for requesting feedback on the system design. I > personally > appreciate this, and I'm pretty sure that the feeling is shared by most of > the > Linux kernel developers. > > > If you have additional questions or need more information please feel > free > > to contact us (we have setup a mailing list at > > vpss_driver_design@xxxxxxxxxxx) so we can answer them. > > I'll answer here as the instructions provided in the wiki to subscribe to > the > vpss_driver_design mailing list are incorrect (http://list.ti.com/ isn't > accessible, the name has no A record associated to it). I've CC'ed the > list in > case subscription wouldn't be required to post. > > 1. Multi-core design > -------------------- > > OMAP3 was already a dual-core system, OMAP4 (I assume all this is about > the > OMAP4 processors family) seems to push the concept one step further. > > With its heterogeneous multi-core design (ARM master CPU and slave DSPs), > the > OMAP architecture delivers high performances at the cost of higher > development > time and effort as users need to write software for completely different > cores, usually using different toolchains. This is in my opinion a good > (or at > least acceptable) trade-off between CPU power, development time and power > consumption (DSPs being more efficient at signal processing at the cost of > a > higher development complexity). > > I'm a bit puzzled, however, by how the VPSS MCU will help improving the > situation compared to the OMAP3 design. The VPSS MCU will provide an API > that > will expose a fixed subset of the hardware capabilities. This is only a > guess, > but I suppose the firmware will be fairly generic, and that TI will > provide > customized versions to big customers tailored for their needs and use > cases. > The "official" kernel drivers will then need to be changed, and those > changes > will have no chance to be accepted in the mainline kernel. This will lead > to > forks and fragmentation of the developers base among the big players in > the > embedded markets. What will be the compensation for that ? How will the > VPSS > MCU provide higher performances than the OMAP3 model ? > > 2. VPSS firmware and API > ------------------------ > > The wiki doesn't state under which license the VPSS MCU firmware will be > released, but I suppose it won't be open sourced. The VPSS API, which > seems > from the information provided in the wiki to mimic the V4L2 API at least > for > video capture and output, will thus be controlled by TI and pretty much > set > into stone. This means future extensions to the V4L2 API that will provide > more control over the devices to userspace applications will be stuck with > access to a limited subset of the hardware capabilities, and users will > not be > able to use the full potential of the system. > > This goes in the opposite direction of what the Linux media community is > trying to do today. For the past 6 months now we have been working on > additions to the V4L2 subsystem to create a complete media framework, > targeted > at both desktop and embedded use cases. The new APIs that we are > developing > will let userspace applications discover the internal topology of the > hardware > and control every parameter in the video pipeline(s). This include dynamic > reconfiguration of the pipeline(s), completely under control of userspace. > With a VPSS API that mimics today's V4L2 API, the OMAP4 video pipeline > will > look from a userspace perspective as an old-school V4L2 device, a single > black > box with a few controls to accommodate common use cases. > > Regardless of the firmware license, we need a way to control hardware > without > any limitation from the ARM processor. This includes explicit > configuration of > the pipeline, and access to all configuration parameters of all hardware > processing blocks. > > 3. VPSS API usage from kernel space > ----------------------------------- > > The wiki mentions that Linux kernel drivers will have access to functions > that > convert "standard kernel data structures" to VPSS data structures as > required > by the VPSS firmware. I don't think that's a good idea. Please let kernel > drivers do the conversion themselves. Linux kernel drivers know about > their > data structures better than the VPSS library/middleware/layer/whatever > will > do. Instead of providing such conversion functions, I would like to see > the > VPSS data structures properly documented so that kernel driver developers > will > know what information the VPSS MCU expects. Filling the VPSS data > structures > from "standard kernel data structures" should be left to individual > drivers > and/or subsystems. > > As explained above, I'm really concerned about the following usage > example: > > "Driver calls VPSS set_format function and passes the VPSS format data > structure. The VPSS set_format function will then: > - Create a message structure for sending over the Notify IPC > - Set the command element with the set format command value > - Set the arguements element to the address of the VPSS format data > structure > - Call the syslink Notify kernel API and send the address of the message > structure to the VPSS" > > This means the VPSS MCU will expose a single black box to the host, making > it > impossible to use the full capabilities of the hardware with future V4L2 > extensions. Those extensions are developed for a reason. V4L2 simply > doesn't > scale in the light of future (and even today's) embedded hardware. If the > VPSS > API mimics V4L2 it will suffer from the same problem. > > One possible solution would be to open-source the VPSS MCU firmware, > allowing > the Linux community to expose capabilities needed by future V4L2 > extensions > through the VPSS API. > > 4. VPSS API usage from userspace > -------------------------------- > > I have no specific comment about the userspace API usage, but I would like > to > know how you plan to arbitrate access to the hardware from both > kernelspace > (through a V4L2 driver) and userspace. Will there be a way for kernel > drivers > to take ownership of specific hardware parts and disallow userspace > applications from issuing any message to those parts ? The design must be > carefully reviewed to spot possible race conditions and even security > issues. > > 5. Syslink > ---------- > > I still need to review the syslink code. As stated by Hans Verkuil, from a > quick look at the source tree the syslink module looks over-engineered. To > communicate with the VPSS MCU all that seems to be needed is a mailbox- > like > interface. > > Furthermore, the mailbox API should probably not be OMAP4-specific. Isn't > there already a mailbox API in Linux ? If not I think one should be > developed > first, and then syslink should be built on top of it. The best way to see > a > driver being rejected when submitted to mainline is to write a huge pile > of > code and then push it in one go. > > > > As a conclusion, I believe that the best chance to get drivers into > mainline > and to get developers excited about the product (both are related in my > experience) is to be as open as possible and play by the rules of the > Linux > kernel community. This means that: > > - Big subsystems such as syslink should be broken down to small pieces, > and > every piece, especially the low-level ones, must be carefully designed > with > the whole Linux kernel in mind, not only the OMAP4 platform. APIs should > be > made generic when possible. > > - The VPSS MCU firmware should be properly documented, developed in the > open > and under an open-source license. > > Those two steps should be performed in tight cooperation with the Linux > kernel > community. > > -- > Best regards, > > Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
