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Diffstat (limited to 'Documentation/video4linux/bttv/README.quirks')
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diff --git a/Documentation/video4linux/bttv/README.quirks b/Documentation/video4linux/bttv/README.quirks new file mode 100644 index 000000000..92e03929a --- /dev/null +++ b/Documentation/video4linux/bttv/README.quirks @@ -0,0 +1,83 @@ + +Below is what the bt878 data book says about the PCI bug compatibility +modes of the bt878 chip. + +The triton1 insmod option sets the EN_TBFX bit in the control register. +The vsfx insmod option does the same for EN_VSFX bit. If you have +stability problems you can try if one of these options makes your box +work solid. + +drivers/pci/quirks.c knows about these issues, this way these bits are +enabled automagically for known-buggy chipsets (look at the kernel +messages, bttv tells you). + +HTH, + + Gerd + +---------------------------- cut here -------------------------- + +Normal PCI Mode +--------------- + +The PCI REQ signal is the logical-or of the incoming function requests. +The inter-nal GNT[0:1] signals are gated asynchronously with GNT and +demultiplexed by the audio request signal. Thus the arbiter defaults to +the video function at power-up and parks there during no requests for +bus access. This is desirable since the video will request the bus more +often. However, the audio will have highest bus access priority. Thus +the audio will have first access to the bus even when issuing a request +after the video request but before the PCI external arbiter has granted +access to the Bt879. Neither function can preempt the other once on the +bus. The duration to empty the entire video PCI FIFO onto the PCI bus is +very short compared to the bus access latency the audio PCI FIFO can +tolerate. + + +430FX Compatibility Mode +------------------------ + +When using the 430FX PCI, the following rules will ensure +compatibility: + + (1) Deassert REQ at the same time as asserting FRAME. + (2) Do not reassert REQ to request another bus transaction until after + finish-ing the previous transaction. + +Since the individual bus masters do not have direct control of REQ, a +simple logical-or of video and audio requests would violate the rules. +Thus, both the arbiter and the initiator contain 430FX compatibility +mode logic. To enable 430FX mode, set the EN_TBFX bit as indicated in +Device Control Register on page 104. + +When EN_TBFX is enabled, the arbiter ensures that the two compatibility +rules are satisfied. Before GNT is asserted by the PCI arbiter, this +internal arbiter may still logical-or the two requests. However, once +the GNT is issued, this arbiter must lock in its decision and now route +only the granted request to the REQ pin. The arbiter decision lock +happens regardless of the state of FRAME because it does not know when +FRAME will be asserted (typically - each initiator will assert FRAME on +the cycle following GNT). When FRAME is asserted, it is the initiator s +responsibility to remove its request at the same time. It is the +arbiters responsibility to allow this request to flow through to REQ and +not allow the other request to hold REQ asserted. The decision lock may +be removed at the end of the transaction: for example, when the bus is +idle (FRAME and IRDY). The arbiter decision may then continue +asynchronously until GNT is again asserted. + + +Interfacing with Non-PCI 2.1 Compliant Core Logic +------------------------------------------------- + +A small percentage of core logic devices may start a bus transaction +during the same cycle that GNT is de-asserted. This is non PCI 2.1 +compliant. To ensure compatibility when using PCs with these PCI +controllers, the EN_VSFX bit must be enabled (refer to Device Control +Register on page 104). When in this mode, the arbiter does not pass GNT +to the internal functions unless REQ is asserted. This prevents a bus +transaction from starting the same cycle as GNT is de-asserted. This +also has the side effect of not being able to take advantage of bus +parking, thus lowering arbitration performance. The Bt879 drivers must +query for these non-compliant devices, and set the EN_VSFX bit only if +required. + |