Hi Greg, while working on video input support for embedded platforms a few developers including myself ran independently into a Linux device model issue. We all came up with hackish solutions that we are not very happy with, and we'd like to fix this in a clean way. The problem comes from devices sitting on multiple busses, a situation commonly found with video sensors connected to an embedded System on Chip (SoC). The sensor is controlled through an I2C bus and sends video data on a parallel video bus, connected to a camera controller usually referred as an Image Signal Processor (ISP), Video Processing Front End (VPFE), CCD Controller (CCDC) or simply a bridge. The bridge and the I2C master controller on the SoC are completely independent from each other. The I2C master controller is not dedicated to the video function and is often used to communication with non-video I2C devices. Unfortunately, on the sensor side, I2C and video bus are not independent. The I2C slave controller usually requires an external clock to be present, and the clock is usually provided on the video bus by the SoC bridge. As the bridge and I2C master live their own life in the Linux device tree, they are initialized, suspended, resumed and destroyed independently. The sensor being an I2C slave device, Linux initializes it after the I2C master device is initialized, but doesn't ensure that the bridge is initialized first as well. A similar problem occurs during suspend/resume, as the I2C slave needs to be suspended before and resumed after the video bridge. Have you ever encountered such a situation before ? Is there a clean way for a device to have multiple parents, or do you have plans for such a possibility in the future ? I would be willing to give an implementation a try if you can provide me with some guidelines. -- 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
